Optional Readonlyclassthe arguments naming S4 classes the call uses (Class in new, signature in setMethod)
Optional Readonlygenericthe argument naming the S4 generic the call answers or asks about (f in setMethod)
Optional Readonlyregistersthe argument naming an S4 class the call adds to without declaring it (to in setAs): read and registered anew, so a later use depends on this call as well as on the declaration
Which arguments of a call name S4 registrations it depends on rather than declares.