A class-A compulsory directive was a standard Starfleet command directive regarding resources for a starship's main computer. The initiation of a class-A compulsory directive involved the use of all necessary computing resources toward the resolution of the specified problem to the exclusion of all other directives, via the use of a compulsory scan unit.
Spock ordered the USS Enterprise's main computer to calculate the value of pi to the final digit using a class-A compulsory directive when the Redjac entity took possession of the computer in 2267. Because pi is a transcendental number with no definitive end, the computer would be continually focused on the task until ordered otherwise. (TOS: "Wolf in the Fold")