How the list below gets filled?

This list fills once you make an exception for a CSS/JS asset (handle) to load through the option "Load it for URLs with request URI matching this RegEx" IF if the targeted CSS/JS file is already bulk unloaded (e.g. site-wide, on all post pages, etc.) by specifying a RegEx. If you wish to add new load exception RegEx rules for other CSS/JS files, access the "CSS & JavaScript Load Manager" for a page that loads the targeted file.