What's the difference between a 'bottom' and a 'sub'?

Like other labels and terminologies, these words might have slightly varied meanings for various people. But generally speaking, a "bottom" is someone who only assumes the submissive role during a play session or scene for a predetermined, constrained amount of time. Throughout the scene, they play the part of the bottom, taking in their partner's actions—who, in this case, would be called the "top."

A "submissive," on the other hand, is typically someone who, depending on what has been agreed upon by the parties, embodies a role of submission within a relationship dynamic, either full-time or part-time. A submissive may view their title as indicating a more psychologically complicated, round-the-clock commitment, in contrast to a bottom, who primarily plays a receiving role in sexual encounters and scenes. When investigating a d/s lifestyle, a submissive and their dominant partner or partners may decide on specific guidelines and frameworks. Naturally, a person can play both dominating and submissive roles. However, it is very typical for one to favor one over the other.

These are merely extremely general definitions, and everyone has a slightly different interpretation of what these terms mean to them. This is true of all labels and language. To understand where people stand and what "bottom" or "sub" means to them, I always advise you to spend some time talking to them about these concepts.