For example, you complete Indoc on Aug 29th, your "base month" is August. You have a "grace period" of a month before, or a month after, which if you train in July or September, the due month can remain august.
If you complete systems on Sept 15, then SEP is your base month.
Your base month may be changed due to company needs, however. In the above circumstance, they might bring you into training in AUG and "rebase" you to AUG.
Same deal with the PC, if it's done in AUG then you have the month prior and after to maintain the base month.