To me it doesn't add that much clarity, I think both valid and domain are a bit vague. The comment in the declaration of the field says it references "array items", but doesn't say what array, and the struct NlaEvalChannel doesn't declare any arrays (it just has pointers and scalar values). If you know what's being referred to, please update that comment as well, because to me it's all a bit muddy.

The clarity was more for consistency with the domain() processing function names and the core struct member name it stores the results in ("valid").

For the comment Mask of array items controlled by NLA, I agree it's useless. A better one may be Associated with the RNA property's value(s), marks which elements are affected by NLA. In this sense, "domain", which implies a function can operate on it, seems more natural than "valid", which implies something is wrong if false.

In D9692#252692, @Wayde Moss (GuiltyGhost) wrote:

A better one may be Associated with the RNA property's value(s), marks which elements are affected by NLA.

So I guess that "the RNA property" means "whatever rna_path points to". If that's indeed the case, your suggested comment is clear.

In this sense, "domain", which implies a function can operate on it, seems more natural than "valid", which implies something is wrong if false.

In that case I agree with renaming valid to domain.