About SWG

You would have to ask that question of the vendors to be certain, but in general, conductors are sold based n the cross sectional area of the conductor material, outside dimensions are considered consequential.

Beg pardon, but did you not mean "inconsequential" ?

Well, since the OP is from India, SWG is more common here, unless he is making something for the US market. Anyway, the enamel thickness becomes a factor when winding coils of fine wire in tight spaces. Generally IEC specs give fine, medium and coarse enamel covering. If winding space is tight, i would use good quality F class wire with fine covering.

Straight answer to your question: "copper thickness".