that index keys that are BinData type are more efficiently packed if:

subtype value is in the range of 0-7 or 128-135
length of the byte array is 0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16, 20,
24, 32

other sizes are fine but these are particularly efficient. the idea is
that say you want to store a UUID. that would be length 16, subtype 4, and
would be stored in a b-tree index more compactly that the pure bson spec's
representation.