The userstartup.cheat file is where you can put custom settings that you want turned on each time you start the game. You can also create "aliases" (shortened commands) to run cheat commands. Anything which is in your userstartup.cheat file will be activated when you run the game.
Because the file lives in your "the Sims 2\My Documents" folder - it allows you to have different custom settings for different user accounts, AND FOR EACH DIFFERENT GAME if you separate them out. So, for example, if you have one version of the game which is mediaeval and one which is apocalyptic, in separate Sims 2 olders, each can have ts own setting for cheats.
In My Documents\EA Games\The Sims 2\Config create a file. Make sure the file is named exactly this way: userstartup.cheat
Right-click on that file and choose "Open With" and then choose Notepad or a similar text-only program. (not Wordpad) to open the userstartup.cheat file in Notepad.
Cheats you put in this file will be automatically called when the game is loaded.
My userstartup.cheat for my Uberhood currently looks like this
boolProp allow45degreeAngleOfRotation true
boolprop ControlPets on
boolprop PetActionCancel true
boolprop PetsFreeWill true