Brezhnev, a "field recording" is simply one made of a traditional performer who is untrained (or in the case of fiddlers, for example, only slightly trained) in "art" music.

A "commerical recording" can mean two things. It can be any recording of anything that's released to the public commercially (including items like the Columbia-Lomax discs, made up, as you say, of field recordings).

Or it can focus on commercial releases of trad music performed in a familiar commercial (rather than a trad) style.

So sometimes "commercial" points to production and marketing, sometimes instead to the style of the performance.