In the old days (50 to 90 years ago), both chip manufacturers (USPC and Burt) referred to all inlayed chips as crest & seal (or "seal & crest" or "crest or seal" or "seal or crest"). That definition was too broad for collecting purposes.

Today, I would define a crest & seal as a predominately clay chip with the following characteristics:
a) The chip has a plain mold
b) The chip has an inlay with printing or a litho on the inlay.
c) The inlay is absolutely even or smooth with the rim surface.
c) The chip has a clear film protecting the inlay.

The above definition would exclude any chip with a mold other than plain (hat & cane, small key, rectangles, arrodie, etc). Plain molds that have an inlay depressed from the rim surface are also excluded. Also excluded are die cuts and hot stamps. Lastly, I would exclde modern chips that appear to be crest & seals but do not have a film covering the inlay.

Roulette chips from the Flamingo and Dunes, for example, are in a gray area. The protective film, if there is one, is extremely thin. Some would call them plain molds, not crest and seals. I am not sure. That's why I collect most all plain molds. I only exclude chips that I consider modern (1980s and later)

The above represents my opinion. Others, such as Steve Passalacqua, Robert Eisenstadt or Allan Myers may have a different definition.