Major I propose changing the terminology used for the metadata fields.
The term "Identifier/Value pair" becomes "Property", and the term "User Defined Name-Value Pair" becomes "User Property". The plural forms would be Properties and User Properties.
This name is more in line with what these sort of things are called in other protocols.
This also makes it much easier to make phrases like "Length of Properties" as opposed to "Length of Identifier/Value pairs" which tends to make the sentence it is in hard to parse.