tbh there is too many different names for it, what he is referring to is the uniqueID/header data/cartID
i havent seen anyone refer to it as "Main Title ID" so i assume your talking about something else probably the Title ID of the game which the save name is set too, this is not the same thing thats just the ID of the game which is the same for all copies of the game (except those from other regions)
just read the first post it explains it all
I've got "Main Title ID" from "Gateway ROM Patcher v0.6", and since I couldn't find "Cartridge ID" I assumed these two were synonymous.