Edit: Wait, refrigeration is just full stack where HVAC isn't.

Yep. When it comes to HVAC and refrigeration, refrigeration techs tend to have far more variried work and be the more expensive to hire. More expensive doesn't mean better though, refrigeration is just more generalized. HVAC tends to be pretty standardized as far as operating conditions and equipment goes so HVAC techs can get pretty specialized as far as tools and training goes. But with refrigeration you do all the standard HVAC work while also working on basically everything else that gets hot or cold.

The best comparison I can make is probably to automotive mechanics. A HVAC tech is like a dealership mechanic, where a refrigeration tech is closer to a diesel truck mechanic. That diesel truck mechanic could probably work on your Honda Civic, but they'd be way more expensive, they aren't going to have the special factory tools, and that really isn't what they're there for. Similarly a refrigeration tech can do HVAC work and we often do, but we tend to be more expensive and less specifically tooled for HVAC than a dedicated HVAC tech. The less specifically tooled part can be kind of funny at points because some of the guys I know who work in dedicated HVAC will go so far as to have different manifolds that they use for different refrigerants, meanwhile I've seen a refrigeration tech patch a 100 ton industrial chiller barrel with basically nothing but the tools in their pockets, a tube of quick set JB weld, and a vacuum pump. If it's stupid and it works then it'll just have to keep working until the right parts come in.