Perhaps protocols/interfaces are like restrictive adjectival phrases, and metadata like non-restrictive. Just in case you don't know: Restrictive clauses limit

Perhaps protocols/interfaces are like restrictive adjectival phrases, and metadata like non-restrictive. Just in case you don't know: Restrictive clauses limit the possible meaning of a preceding subject, whereas non-restrictive clauses tell something about a preceding subject, but do not limit its meaning. Example of restrictive use: The suspect in the lineup who has red hair committed the crime. Example of non-restrictive use: The suspect in the lineup, who owns a red car, committed the crime.