Why HMS Unicorn counted as an aviation support ship

Why was HMS Unicorn classified as an auxiliary rather than an aircraft carrier?

She was purpose-built for forward aviation support, and the auxiliary classification prevented her from consuming treaty carrier tonnage.

HMS Unicorn was explicitly designed to meet the criteria for a forward aviation support ship. The auxiliary classification kept her outside the aircraft-carrier tonnage allowance. Classifying her as a carrier would have charged her displacement against that restricted category. The answer therefore treats the support role and the tonnage account as connected parts of the design decision.