GroupSpecification

The argument of $group: an _id that says how to group, plus any number of output fields, each computed by an accumulator.

_id is a field reference such as "$team", a literal (null puts every document in one group), or a document of up to 8 field references or literals for a compound key. Two documents are in the same group when their keys are equal as filters see equality, so 1 and 1n are different groups. Groups come out in the order their first document arrived, and at most 100,000 groups may exist at once.