One of the more frustrating aspects of observing special counsel Robert S. Mueller III's investigation is that we don't even know how much we don't know. Apart from anonymously leaked tidbits, we don't see much, and Mueller and his team don't speak publicly.

But sometimes, because of legal proceedings, we get a window into what's going on.

That's what happened with a Monday night filing in the case against onetime Trump campaign chairman Paul Manafort.

Manafort's lawyers argued that Mueller traveled outside his lane in focusing on Manafort's financial dealings, so the special counsel was forced to respond with a detailed defense of the scope of his investigation. It clocks in at 282 pages.

The big takeaway is that Mueller received permission from Deputy Attorney General Rod J. Rosenstein in August to look into Manafort's work for the Ukrainian government.

But there is plenty more of substance in the filing. Below are a few takeaways.

Mueller was diligent about seeking permission

Here's the section referenced above:

The process of defining the Special Counsel’s jurisdiction has also included nonpublic dialogue between the Acting Attorney General and the Special Counsel. In particular, on August 2, 2017, the Acting Attorney General issued a memorandum about “[t]he Scope of Investigation and Definition of Authority” conferred on the Special Counsel. Memorandum from Rod J. Rosenstein to Robert S. Mueller. ... As relevant here, the memorandum specified that the following allegations against Manafort “were within the scope of [the Special Counsel’s] investigation at the time of [his] appointment and are within the scope of the [Appointment] Order”:

What's perhaps most notable here, I would argue, is that one might assume that investigating Manafort's work for Yanukovych and Ukraine was within the scope of the investigation. After all, Yanukovych was heavily tied to Russia and Vladimir Putin, and this investigation is about Russia. Yet Mueller didn't assume anything.

Around the same time Rosenstein issued this memo, he said publicly that Mueller needed to ask him for such permission to expand the scope of the investigation. “Bob Mueller understands and I understand the specific scope of the investigation, and so no, it's not a fishing expedition,” Rosenstein said.

This suggests that Mueller has indeed been seeking permission to expand the inquiry whenever there has been a question. This would seem to insulate Mueller from charges that he has overstepped his mandate — and suggest that he has taken great care to guard against both that perception and legal argument.

Rosenstein's memo is heavily redacted and appears to include other approved expansions of Mueller's scope

The section above actually spans some heavy redactions from Rosenstein's memo — redactions that appear to be other areas in which Rosenstein gave Mueller permission to expand the investigation.

Here's how the memo physically looks:

Everything that is redacted follows behind this phrase: “The following allegations were within the scope of the Investigation at the time of your appointment and are within the scope of the Order.” That suggests that at least the first big black box — if not the even bigger, second black box — contains other things Mueller has Rosenstein's approval to investigate.

Exactly what things lie behind those black boxes, we don't know. They could be small and noncontroversial, or they could be big aspects of the investigation and/or things we don't know about. But there do seem to be plenty of things — which suggests a potentially broad berth Rosenstein has given Mueller.

Mueller is investigating Manafort not just to flip him, but to see if he 'colluded'

One word in the above section stands out perhaps more than any other: “Colluded.”

Manafort's ties to Russia and its ally in Ukraine are well-known. But that's a long way from suggesting that there was collusion. And there has arguably been more public evidence that Donald Trump Jr. attempted to collude with that meeting with a Russian lawyer and that Jared Kushner and Roger Stone tried to communicate with Russia.

But court filings increasingly are linking Manafort to possible collusion as well. Last week, another filing described a Manafort business associate in Ukraine as having had ties to Russian intelligence during the 2016 campaign. And this filing makes clear Mueller was seeking to answer whether Manafort might have colluded with a country he was once indirectly in business with. It doesn't say how compelling that evidence is, mind you, but it makes it crystal clear that was a focal point of the probe.

Up until last week, what we knew publicly suggested Manafort might be targeted in hopes that he would flip on Trump rather than for possible collusion. That's changing.

Manafort's defense just took a blow. Will he cut a deal now?

Manafort is the one major figure in this case who is facing charges but hasn't cut a deal. His business partner Rick Gates has, as have fellow former Trump advisers Michael T. Flynn and George Papadopoulos. Exactly why Manafort hasn't reached a deal in the face of dozens of corruption charges — he even has a trial date set — is the big question.

To the extent that Manafort's defense is predicated on Mueller having overstepped his authority, that seems to be out the window; Rosenstein gave Mueller pretty explicit authorization to look into this, and Rosenstein is the guy in charge here as the acting attorney general (following Attorney General Jeff Sessions's recusal).

Perhaps this was just a failed effort — a Hail Mary — by Manafort's legal team to get the case thrown out or to get a glimpse of Mueller's hand. But it also seems possible that this will increase the pressure on Manafort to cut a deal.