{('PX5'),('PX6')}
looks like a relation. But I also don't understand your term. A rename is defined as ρa/b(R).
So the best what I could think of your term would be something like ρx/code({('PX5'),('PX6')}), which means:
Rename the attribute 'code' into 'x' of the relation {('PX5'),('PX6')}.