That 5.5 percent executive pay raise Congress cleared is what is known in the federal pay raise trade as a "pure" increase. The difference between "pure" and "regular" is, in this case, about 3.4 cents is in addition to the $2,600 annual raise that they expected.

Under the new pay scale as approved, top grade federal officials will be permitted a 5.5 percent raise up to the new maximum salary of $50,112.50. The old pay ceiling was $47,500. Normally the federal pay rate would have been rounded off to give Grade 18 and equivalent personnel $50,100.

But the language of the confused congressional pay authorization did not allow for rounding, so executives will be at a scale $12.50 a year more than they had bargained for. At this stage, the frustrated, long-frozen officials will take anything they can get.

The executive pay raise was approved as part of a continuing resolution authorizing agencies to stay in business pending final approval of their budgets. Although parts of the resolution expire next month, the pay raise is authorized through the remainder of this fiscal year. It ends Sept. 30, 1980.

Under the pay scales, now in effect, all employes in Grades 17 and 18, and most Grade 16 government executives will move up from $47,500 to the new $50,112.50 level. Senior employes in Grade 15 also will move up to that salary.

The first step of Grade 16, however, will pay only $47,889 because that is the way pay tables work out. Since a limit remains on career federal pay, some employes will not get full 5.5 percent raises. They can get up to $50,112.50 but no more.