$_internalBucketGeoWithin (SERVER-57762) selects buckets that might contain events within a given region. That is:

• It is guaranteed to return every bucket that does contain an event in the given region.
• It may return other buckets, depending on the query plan.

$geoWithin queries on events use $_internalBucketGeoWithin to narrow down the set of buckets (before unpacking and doing a more precise filter on events). As of SERVER-57763, partial indexes can use $geoWithin in partialFilterExpression. To make these features work together, we need to allow $_internalBucketGeoWithin in the partialFilterExpression.

I think the rule for query containment is the same as a plain $geoWithin. If the index has regionA, and the query has regionB, then the index can be used only if regionB is contained in regionA.