Running with recent posts here and some discussion on the matter on /smtg/ threads past, the "lewd" tag is being deployed with about the criteria you'd expect.

rating:safe - Image contains no explicit or implicit sexual content.
rating:questionable - Image contains suggestive sexual content and/or incomplete exposure of above-the-belt sexual organs. This is the minimum at which Mara may be depicted, and even then with considerable "softening" of the design.
rating:explicit - Image contains fully exposed sexual organs, but not necessarily their use in sexual contexts. Uncensored Kaneko art (Diana, Tiamat, etc.) typically goes here.
rating:explicit + lewd - It's porn, son of man.