Field help (ABTHOR) 

This code is used to associate an execution time constraint on the task to limit the possible execution times when they are directly submitted to the batch server.

When is submitted by a group of tasks, only the time constraints attached to the group are applied.

It is the same case for a recurring task : the execution times are not then controlled with respect to the task constraint code, but an exclusion calendar is defined directly in the recurring task).