Leather is a flavor word, not a wrapper type. People reach for it when the smoke feels tannic, dry-sweet, or like a belt or a chair — the way they reach for cedar when a box has been speaking. It shows up on dark wrappers and on some sun-grown habano covers, which does not make it a rule.
Smoke Rings uses the word as English, not as a branded tasting system. If we cannot say it plainly, we leave it off.
See earth and cocoa for the other two seed notes in this glossary, and the reviews for cigars spoken about as cigars.