If your church or denomination requires a course besides CM training (say, Safe Sanctuaries), add it under Settings → Rules → Other certifications needed to serve: its name, the People custom field its date is kept in (the field's exact name), and how many months it's valid. Arkrooms reads that date with the roster, the same way as the CM Training Date, and never stores it. Someone with no date, or whose date has run out, isn't scheduled, and the serving record for your insurer shows each certification's status on each Sunday served. Planning Center's own setup is up to you: the field must be a date, and the connection needs to see its tab in People.
As with CM training, the volunteer, their team's leaders and the admins hear 60, 30 and 7 days before one runs out, and on the day; and the weekly leader email lists who must renew or record one before the team's next turn.