Looks to me the guidance is quite clear, they can designate 40% of the published timetable (not a slimmed down P-coded version), and issue work notice. The exact calling pattern doesn't have to be same as published timetable
Can add others to the work notice until 4 days before (if for instance somebody on work notice called in sick),
As for which services to choose, the guidance says to enable passengers to make important journeys, to access work, healthcare and education, and also to protect the wider economy eg retail and night time economy).
So that seems to me couldn't just work peak hours, instead of say, a school train, or one serving late night shopping under the guidance