In SUD, the subj relation is used for all subjects (either nominal which are nsubj in UD or clausal which are csubj in UD).
Hint
Most of the times, subj is used without extension.
Possible extensions are:
subj@explsubj@caussubj@pass
Other extensions appear during conversion from UD ⇒ SUD, UD subrelation are kept as “deep” SUD relation for information preserving. For instance UD:nsubj:cop ⇒ SUD:subj@cop.