I struggled with the naming here. These are the RVs that the model is parameterized in terms of. This is distinguished from deterministics which are also random variables but the model is not parameterized in terms of them. 

There are three kinds of variables: free, observed, deterministic. There are 7 combinations:

free 
observed 
deterministic
free + observed 
free + deterministic
observed + deterministic
free + observed + deterministic 

I'm open to better names. It would be nice to have good names for each of the ones that get used.