If your Emacs was built with the tree-sitter library, it can use the results of parsing the buffer text by that library for the purposes of fontification. This is usually faster and more accurate than the ``traditional'' methods described in the previous subsection, since the tree-sitter library provides full-blown parsers for programming languages and other kinds of formatted text which it supports. Major modes which utilize the tree-sitter library are named /foo/-ts-mode, with the -ts- part indicating the use of the library. This subsection documents the Font Lock support based on the tree-sitter library. You can control the amount of fontification applied by Font Lock mode of major modes based on tree-sitter by customizing the variable treesit-font-lock-level. Its value is a number between 1 and 4:

Level 1

This level usually fontifies only comments and function names in function definitions.

Level 2

This level adds fontification of keywords, strings, and data types.

Level 3

This is the default level; it adds fontification of assignments, numbers, etc.

Level 4

This level adds everything else that can be fontified: operators, delimiters, brackets, other punctuation, function names in function calls, property look ups, variables, etc.

@noindent What exactly constitutes each of the syntactical categories mentioned above depends on the major mode and the parser grammar used by tree-sitter for the major-mode's language. However, in general the categories follow the conventions of the programming language or the file format supported by the major mode. The buffer-local value of the variable treesit-font-lock-feature-list holds the fontification features supported by a tree-sitter based major mode, where each sub-list shows the features provided by the corresponding fontification level. Once you change the value of treesit-font-lock-level via M-x customize-variable (Specific Customization), it takes effect immediately in all the existing buffers and for files you visit in the future in the same session.