14 CFR §21.175

1. (a)Standard airworthiness certificates are airworthiness certificates issued for aircraft type certificated:
   1. (1)In the normal, utility, acrobatic, commuter, or transport category;
   2. (2)As manned free balloons; or
   3. (3)As special classes of aircraft.
2. (b)Special airworthiness certificates are airworthiness certificates issued for:
   1. (1)Aircraft type-certificated in the primary, restricted, provisional, or limited category;
   2. (2)Aircraft certificated in the light-sport category;
   3. (3)Aircraft operating for an experimental purpose; or
   4. (4)Aircraft operating under a special flight permit.