24.6.5 Levels of Font Lock

Some major modes offer three different levels of fontification. You can define multiple levels by using a list of symbols for keywords in font-lock-defaults. Each symbol specifies one level of fontification; it is up to the user to choose one of these levels, normally by setting font-lock-maximum-decoration (see Font Lock in the GNU Emacs Manual). The chosen level’s symbol value is used to initialize font-lock-keywords.

Here are the conventions for how to define the levels of fontification: