Such a type will be used to disambiguate "paths" like "a.b" and "a.0.b". For example, you could imagine translating "a.b" into a path "start at root, go through a subdocument 'a' to find field named 'b'" or "start at root, go through each element of the array a and look at the field 'b' within each of those documents". "a.b" might mean either in MongoDB.