The capacity a parent and one of its children share, as two orthogonal facts:

• staticCap — the group's structural ceiling (groups.max_attendees), date-INDEPENDENT. A share whose static cap is below PARENT_CHILD_GROUP_UNITS can NEVER fit a parent+child order, on any date — so date-less surfaces can mark it sold out without a date.
• remaining — the group's currently-free spots in the caller's context (date-less cumulative for standard listings; per-date when a date is known; undefined when not computable, e.g. a daily child with no submitted date).

Both are undefined when the parent and child do not share a capped group. This is the single capacity vocabulary the bookability evaluator reasons over, so every surface answers "does the combined demand fit?" the same way.