RefID is a wrapper type around a ulong value which represents a pointer to a world element.
It uses it's last 2 bytes to represent the allocation id of a user who allocated the world element, and the remaining bytes are used to represent which world element it's pointing to.
It is primarily an internal engine type and is not meant to be used directly.
Many users eventually found that it is useful for Ref Hacking, as this type is a reference to different parts of the engine (Slots, Components, etc)