This is documentation about a configuration directive, which can be placed within Movable Type’s core configuration file,
mt-config.cgi, to customize the behavior of the system.
The IncludesDir configuration directive can be used to override the default directory into which MT will cache SSI files.
The value must be a path relative to the site root or an absolute file path.
includes_c — the “_c” is short for “cache” in the hopes that this directory doesn’t conflict with other directories in the published blog/site.