As Al explains, one is for buildings and one is for vehicles, also.
The TACOM Code was developed from AWS D1.1 as well as others. TACOM includes, and many metals (stainless, steel, armor, etc.) where D1.1 is mainly steel. The PQR’s, WPS’s, and Welder Qualifications are nearly identical (DOUBLE CHECK!!!).
I sometimes can get my customers to accept documents from one or the other if I’m able to cite and explained the differences to them.
Keith