Here are three rather important things that government workers and military personnel ought to bear in mind as they ponder the prospect of an automatic, 6.5 per cent cost of living raise for October:

1) It isn't automatic.

2) It won't necessarily be for 6.5 per cent.

3) It will not be a cost-of-living raise.

The pay machinery the government uses to boost civil service and military pay rates remains a mystery to most people - those who get it, and those who pay for it - even though it has been functioning for nearly a decade.

It's isn't because people are dumb, but rather because the system is so complex and controversial that only people who get paid to understand it usually master the rules. Still it amounts to the biggest payroll outlay in the nation - if not the world. Each 1 per cent increase costs half a million dollars.

The raises are not automatic. The law says that adjustments will be made each October. But the President can - and has in the past - cut back or attempt to delay the amount of the increases due government and military personnel.

The 6.5 per cent figure that is being mentioned comes from both the Ford and Carter budgets. But that is a projected amount, not the final figure. It could be more, or less or the raises could be staggered as they were in October, 1976 when lower-grade people got smaller increases than uppergrade workers.

President Carter won't set the amount of the pay raise until sometime in August. He will do it on the basis of a survey - to be made later this month - by the Bureau of Labor Statistics.

BLS matches selected private industry job pay levels with counterpart positions in government. It comes up with a recommended range which the President - in consultation with unions, agencies and private groups - puts into effect. He can modify the amount, or delay the raises, unless Congress specifically objects. And Congress has already voted to out itself out of any October pay raise this year.

Newspapers frequently refer to the October adjustments as cost-of-living raises. They are not. The increases are based on actual wage gains in private industry. Those private industry increases may take living costs into account, but the raises government workers get are not based strictly on cost-of-living raises.

In recent years, federal-military pay increases have generally been less than the actual cost of living. The 1976 raise, for example, was an average 4.83 per cent while the COL for the period was higher.

So, odds are very good that there will be a some kind of pay raise in October. But it won't necessarily be 6.5 per cent, and it won't be a cost-of-living raise.