Custom is a related term of demand.
In legal|lang=en terms the difference between custom and demand
is that
custom is (legal) long-established practice, considered as unwritten law, and resting for authority on long consent; usage see usage, and prescription while
demand is (legal) to issue a summons to court.
As nouns the difference between custom and demand
is that
custom is frequent repetition of the same behavior; way of behavior common to many; ordinary manner; habitual practice; usage; method of doing, living or behaving while
demand is the desire to purchase goods and services.
As verbs the difference between custom and demand
is that
custom is (obsolete|transitive) to make familiar; to accustom while
demand is to request forcefully.
As an adjective custom
is made in a different way from usual, specially to fit one's needs.