The Implant Size (0068,6210) is the descriptor defined by the manufacturer to distinguish between different sizes of one kind of implant, e.g. size S/M/L/XL.
The Frame of Reference UID (0020,0052) Attribute is used as an identifier for spatial registration in 3D. The Frame Of Reference UID uniquely identifies the spatial frame of reference of all 3D models contained in an Implant Template UID Instance.
If the Frame Of Reference UIDs in two instances are identical, the contained 3D Models are defined in the same 3D space, i.e. require no additional registration.