The jobspec provides the template for creating and editing jobs. The comments in the jobspec describe each field, including what it's used for and its possible values.
To view the jobspec, select Show->Jobspec Detail while viewing any job.
Only a Perforce superuser can add or edit these job fields. It is the superuser who enters the comments about the individual fields into this form. For more information on editing the jobspec, see the Perforce Sytem Administrator's Guide.