The capacity a parent and one of its children share, as two separate facts:
staticCap—groups.max_attendees, date-INDEPENDENT. Below PARENT_CHILD_GROUP_UNITS a parent+child order can never fit on any date, so a date-less surface can mark it sold out.remaining— free spots in the caller's context,undefinedwhen it is not computable, such as a daily child with no submitted date.
Both are undefined when the two share no capped group.