Lua user data are special types used to expose native data into the Lua environment.
There are two types of user data, normal user data which is arbitrary binary data that is garbage-collected, and light user data which is raw memory pointers which are not garbage-collected.
Scaleform Studio uses normal user data to expose handles to C++ objects to the Lua environment and also uses light user data for short term passing of other non-tracked memory between C++ objects.