A profile defines the physical characteristics of a keycap set.
Keyboards may come with any number of differently shaped keycaps. However, keycaps are typically made by reusing existing molds instead of inventing a unique profile for every keyboard or keycap set. This is partly due to high manufacturing costs, but also because designing a high quality keycap profile takes a lot of time, effort, skill, and iteration. Within the enthusiast keyboard space, a small number of keycap profiles have stood out. Most custom keycap sets are manufactured in these community favorite profiles.
Keycap profiles tend to be labeled using two or three letter abbreviations.
These abbreviations are often derived from internal specifications, such as machining standards that they were produced to meet. Or they might stand for a more typical user-facing product name. Regardless, the keyboard community generally refers to these by their abbreviations rather than worrying about that the letters actually mean.