Each user can have their own bespoke keys set on their extensions. So when the default map is set to user programmable keys then they can program these individually as they wish. Speed dials, blf keys, that sort of thing. Alternate keymap can be for when more than one person uses the phone, they can switch to their keymap. Or, if they do not have many keys they can add extra keys to the alt keymap and switch between the two. Can act as two pages of keys.