It is right keys for RT 3.0.x(if I remember correct).
Principals table shre ids with Users and Groups table. You couldn’t have
group id equal to users id.
Creator and LastUpdatedBy are common fields for many tables in RT and
all this fields are reference to users now, but in generic situation
when you take advantage from id sharing fields should refer to the
principals table. RT code jump directly to users object and skip
principals table when you call CreatorObj method for example and IMO
this subject to change.
This is why I’ve done exactly this keys.