A demarcation dispute is a dispute between (usually) two trades unions as to who should do a particular job, and is associated with multi-unionism in an enterprise, where two labour unions claim the right to represent the same class or group of workers.
A demarcation dispute in the UK is a dispute between two trades unions as to who should do a particular job, and is associated with multi-unionism in an enterprise. Elsewhere this may mean two labour unions claiming the right to represent the same class or group of workers.