"mp" is "multi-game-persistence" meaning those SHOULD be variables that get saved to be loaded in a potential sequel.

Most "multipersistent" variables for the purposes of modding just flat-out don't work in AWSW for some reason, so other than mp.save() there's absolutely zero reason to use them.

Regular "persistent" variables are variables that get saved and not reset on a new game start. This is what enables characters to remember certain things that happened in previous playthroughs, these are generally used for much of the same purposes in mods as well.