Democratic Presidential candidate Bernie Sanders takes part in the Milford Labor Day Parade on Sept. 7, 2015, in Milford, N.H. (Charles Ommanney/The Washington Post)

On Thursday, Bernie Sanders received the endorsement of the American Postal Workers Union. "It’s time for a political revolution," the union's president said in a statement. It backs Sanders because he "doesn’t just talk the talk. He walks the walk."

When you see that, what is the impression you get? Is it that Sanders just scored a huge coup? That Sanders is vying for the labor vote? That you have no idea what to make of it?

All natural responses. But we are by now far enough along in this election cycle to clarify things a bit. So for the uninitiated -- and  really even the modestly initiated -- here's a guide to the role of labor endorsements in politics.

The unions

Before reading any further, think of the names of a few unions. Here, we'll add a picture so that you can think of some before we list a bunch.

Democratic presidential candidate Hillary Clinton speaks to Culinary Union members gathered in front of the Trump International Hotel & Tower Las Vegas. (Joe Raedle/Getty Images)

So you thought of the Teamsters, obviously. And probably the postal workers, given that you are reading this article. Who else? Maybe the cops and firefighters? Maybe -- maybe! -- SEIU? Maybe you thought of the AFL-CIO, too?

Don't be embarrassed. There are an inordinate number of unions, mostly organized around particular trades -- hence "trade unionism." The idea, back in the unions' heyday, was to organize a sector of workers across the board so that if an employer needed, say, a welder, they'd have no choice but to hire a union welder and pay union welder wages. That structure (if not the density of union membership) is largely the case today.

So you have teachers' unions like the National Education Association (NEA) and the American Federation of Teachers (AFT). You have the Service Employees International Union (SEIU) and the United Food and Commercial Workers (UFCW). The International Brotherhood of Electrical Workers (IBEW) and the United Auto Workers (UAW). There's the Communications Workers (of America, CWA), the United Association (plumbers and pipefitters, UA) and the Laborers' International Union of North America (LIUNA).

Then there's the grand-daddy of all acronyms: the hotel and textile union, UNITE HERE, formed with the merger of the Union of Needletrades, Industrial and Textile Employees (UNITE) and the Hotel Employees and Restaurant Employees (HERE). UNITE HERE is also a good example of how unions have branched out a bit, blending a few very different trades. The Teamsters (technically the International Brotherhood of Teamsters), for example, don't just organize truck drivers; they organize anyone and anything they can, which of course angers the other unions. (When a union tries to steal already organized workers, this is called "raiding.")

Notice that there are two teachers' unions, who work in competition. Hillary Clinton's already gotten the endorsement of both the NEA and the AFT. In lots of trades, multiple unions compete for members. For example, Clinton could still get the endorsement of the postal workers -- at least, of the National Association of Letter Carriers, a different union than the APWU that backed Sanders. The NALC is also larger, which is beneficial since the union will often send out mail to its members listing its endorsements. If she did get NALC's backing, both Clinton and Sanders could legitimately claim to be endorsed by postal employees.

The hierarchy

Then it gets even more confusing, because there are multiple levels within each union that function as their own organizations.

There's the national or international union (generally called the IU), which is the overarching organization. Then there are state bodies, often called state councils, which have umbrella authority over a state. (Adding to the confusion, the state bodies can have different names than the national organization.) Below that are local unions, which deal with union business at the local level. Locals are usually but not always identified by numbers. The Ironworkers Local 40 is in New York City, for example. Local 377 is in San Francisco. There's often no rhyme or reason to the numbers, so it's hard to necessarily know where a local is based only on its number.

Overarching most of this is the AFL-CIO, which is an umbrella organization of all unions. Not all unions are members of the AFL (as it's known for short). Some have always been independent. About a decade ago, a number of major unions split off from the AFL to form a new umbrella called Change to Win. (The Teamsters, SEIU and the Farm Workers -- did you name the Farm Workers earlier? -- are the current members.) The AFL also operates its own state and local umbrella organizations, in part to be the unified voice of a very splintered system.

And there's another umbrella organization that just covers the building trades. It is called (wait for it) the Building Trades. It, too, has state and local organizations to facilitate activity among its members. If the steelworkers want to strike against an employer, for example, it needs authority to do so -- but it also needs support from other trades that might be working the same project. The Trades (as it is known) manages that stuff.

These are just general guidelines for understanding what's going on. Some local unions encompass large states. Some unions use different hierarchies. It's very messy.

The endorsements

Which makes the endorsements messy, too.

Clinton has gotten the endorsement of the National Education Association, but she does not have the endorsement of the NEA in Vermont, which went for Sanders. SEIU Local 560 (New Hampshire) backed Sanders, but the state SEIU hasn't. Sometimes, state bodies let locals endorse whomever they want; sometimes they don't. Same with the international unions: Some keep a tight rein (or try to). Some don't (or can't).

The AFL would rather everyone endorse the same candidate across the board, because that makes the endorsement much stronger. But the interests of the American Federation of State, County and Municipal Employees (AFSCME) and the International Association of Fire Fighters (IAFF), for example, in many cases diverge. There is only so much public funding to pay so many employees, and so AFSCME and the IAFF are often at odds.

Also at odds are the economic and political aims of the unions. If there were one, solid labor endorsement around which all union members rallied and for whom all unions worked, labor would be much more powerful. But in matters of economics, local specificity is more important. A New York union might need to back a Democrat, while a Mississippi one (if there is one in Mississippi) might need to back a Republican. That relationship with members and the ability to pressure local politicians to support strikes or move policy as needed is more important to the local.

An aside

The conflict between unions is not only about politics. It comes up during membership drives (as mentioned above) and during strikes (some unions might back a business that another union wants to picket).

It also comes up on policy. During the debate over the Keystone XL pipeline, the Building Trades unions backed building the pipeline because their workers would be the ones to build it. But a group of five prominent unions concerned about the environmental effects opposed it. It can be hard, then, to figure out the "labor position" on an issue, just as it is to figure out the "labor candidate." In the Keystone case, the "labor position" ended up being the AFL trump card: It supported the pipeline -- after a delay.

The value

From the candidate side, the value of an endorsement is often situational. Sanders is happy to have the endorsement of (some of) the nation's mailpeople because he needs to show movement against Clinton. Clinton, I can safely assume, doesn't really care. She would be much more worried if she had lost the NEA, in part because the NEA is far larger -- millions of members versus the APWU's couple hundred thousand -- and because the NEA is much more politically active.

That's a critical consideration. Some unions don't do a whole lot when elections roll around. Some unions only do things in certain places, meaning that some locals (or states) are a lot more politically active than others.

A great, 2016-important example is the Culinary Workers in Nevada. The Culinary union (as it is known) is part of UNITE HERE, but has its own discretion to endorse as it sees fit. It is very politically powerful in Las Vegas, thanks to the hotels it has organized, and Vegas is critical to Nevada. That makes the Culinary's endorsement in that early state probably more important to a candidate than the national UNITE HERE endorsement.

A savvy candidate will figure out which unions are most likely to offer support that extends beyond an endorsement. SEIU often is very active in recruiting people to knock on doors or call voters, making it an important endorsement to have in a lot of places. Other unions hardly do anything, but their endorsement might be so resonant that it's important to get. (The textbook example here is the police.)

The moral of the story

Labor endorsements admittedly mean less than they once did, thanks in part to the decline of the private sector union over the past 50 years. But occasional endorsements can be politically significant in particular places and for particular purposes. If you're not paying particularly close attention to labor politics, the meaning of an endorsement might be unclear.

There's an easy shorthand I will share (that will probably make union members angry): Most labor endorsements usually don't mean much of anything. The APWU says Sanders walks the walk, but if its members don't walk precincts for Bernie, that announcement is about all of the good Sanders will get from their endorsement.

Correction: The UFCW rejoined the AFL-CIO and is no longer a member of Change to Win.