Settings for w3m's siteconf. It allows you to match on a url pattern and do various things like url substitutions, site-specific user agent settings, specifying charset, and a few others.
See https://git.sr.ht/~rkta/w3m/tree/master/item/doc/README.siteconf for documentation and examples.
As of w3m v0.5.5, siteconf doesn't respect the W3M_DIR environment variable, so unless programs.w3m.settings.siteconf_file is set, siteconf will always be at ~/.w3m/siteconf.