Until such time as regulation mandates a U.S. Core version higher than 3.1.1, the Physical Activity IG will follow the approach of Gravity, Da Vinci and others of setting the base interoperability expectation to what's mandated in regulation.  Implementers are free to pre-adopt the extensions defined in future versions of US Core if they wish.  Doing so will not negatively impact implementation of the IG.  However, forcing a higher version than regulation requires may create an unnecessary barrier to adoption. That said, we will call attention to the fact that there are useful elements in future versions (including occupation information) that might have relevance to the physical activity space.  As well, we will revisit the version of US Core the IG is based on in the future based on evolving regulation expectations and Gravity SDOH choices.