I should've submitted it here earlier (before writing a tutorial), but I just found this section today. This is sort of collaborative with Kazesui, but I've done most of the newer work was done by me.
I've made a few updates since the original version. Beyond naming items/skills, and working with strings (which is useful for passwords, and steal systems), you can store stuff into files (which can be used for stuff like GamePlus or unlockable bonus content), and arrays (up to (50)(1000), which is quite a few storage spaces), and do a number of processes with extra variables not normally included in the game.
You could have 1000 (well, strictly speaking, 50k) such doors and still only use one variable. This doesn't work well with page switches/variables, because there isn't a way to check for the array until after the event starts. (Parallel processes that control the entire screen have the same problem) But other than this, there are tons of ways to use this. You can make an item storage event, for instance.
Note: I'm splitting this into two parts, so I can add new stuff. The second part is moved to file_control.