Optional Readonlycontainsthe argument carrying the direct superclass(es): S4 contains, S7 parent, R6 inherit, RC contains
Optional Readonlyinstancethe names the methods of the class see the instance they are called on by (R6 self and private, RC .self), so writing through them changes that instance
Optional Readonlymemberthe arguments carrying members, in the order they are to be read
Optional Readonlynamethe argument naming the class
Optional Readonlyoldthe S3 class vector to register as S4 classes (setOldClass), and the arguments that make the classes of an object decide instead
Optional Readonlyprototypethe argument carrying the member defaults (S4 prototype)
Optional Readonlyrelationthe declaration relates existing classes rather than declaring one
ReadonlysystemOptional Readonlyunionthe argument listing the members of a class union (setClassUnion)
Optional Readonlyvaliditythe argument carrying the function checking every new instance (S4 validity)
Optional Readonlyvirtualthe argument saying the class cannot be instantiated (S7 abstract)
How one built-in declares a class, so no processor has to guess an argument's meaning from its position.