applschd.p applies/deapplies a student schedule entry to all the related master schedule records. It does no verification of fitness of the schedule record you pass (i.e. seat counts aren't checked/honored), it just records the course.
The passed stu-schd buffer must have, at a minimum, the following fields filled in:
stu-seq
school-year-id
schd-version-id
bldg-id
course-id
section-id
stu-schd-seq
If there is an associated request, then request-seq should be filled in.