Basically, some external code can choos a unique id and create a message table for itself. The idea is that modders can hook into this as well so they can get their own message table and don't have to worry about conflicting with the base game's textIDs. I have also implemented this setup for Randomizer so that the pre-filled bottles (which didn't exist in the original outside of Ruto's letter) have unique text as compared to the text for the item they are filled with.
1ed45e1433