The documentation seems to go back and forth between using key and collection for the same concept. For example, the syntax for WITHIN is the following:
WITHIN key [CURSOR start]
but then the description states:
WITHIN searches a collection for objects
Another place where this is confusing is in the prometheus metrics where it uses collection e.g. tile38_collection_(points|objects|strings|weight_bytes)
My preference is to use collection in place of key because a collection is plural and represents a group of objects.