System include paths are standard locations to find source code tags, such as the header files in /usr/include and its subdirectories on Unix-like operating systems.
You can add and remove system include paths using the following commands:
Prompts for a directory, dir, and add it as a system include path for the current major mode. When called non-interactively, the major mode can be specified with the mode argument.
Prompt for a directory, dir, and remove it from the system include path for the current major mode (or mode).
Customize the system include path for the current major mode (or mode).
Include tags implied for all files of a given mode. You can set this
defvar-mode-local for a particular mode so that
any symbols that exist for all files for that mode are included.