per file (e.g. key only at ABC.pbo)
per group of files (e.g. ABCD.pbo and ABCDE.pbo) e.g. all addons from addon maker XYZ
and this may be depending on content of file or multiple files
so You can allow e.g. only latest version of addon or all versions of addon ...
that's why it's nearly impossible to share information about signatures use cause even if e.g.:
+ it could be possible to share ALL filenames inside Keys directory but that would on addon heavy servers be more than 1k characters (128 average)
then still it tells nothing about what exactly it allows (just roughly informative)