The first one is the new Customise Tab, used to allow enabling/disabling of extensions, installing of styles and languages.
The second one is ACP_CAT_DOT_MODS used by mods in 3.0 and the first extensions in 3.1 to add their configuration modules.
My idea was to merge them into one
Everything is in one place.
As for the tab, I would keep the language key 'ACP_CAT_DOT_MODS' to allow compatibility for install scripts of mods and migrations of already existing extensions. The translation should be changed to Customise, so it matches the fact that styles and translations are there aswell.
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