The naming convention that we've been following is:
"Custom Keycap Set" = all the kits specified by the designer, e.g. novelties, accent kits, spacebar kits, etc.
"Keycap Set" (no "Custom") = small number of kits tailored to specific keyboards, e.g. Base kit, TKL kit, 65% kit, Ortho kit, etc. (varies with specific set)