Screen type is a number 0 through 255 that organizes trigger effects across the various screens. So if you have two screens both with monster lock tiles and both are screen type 1, then activating the monster lock on one screen will automatically unlock on the other. If they have differnt screen types, then you must perform the monster lock action on both. I hope that helps.
There may be a way to call that value from the code, but I do not know how.