Class versus Instance methods in Program 9.3

[StanleyRShapiro]

Can someone explain how the particular line of code works:

if([Square respondsToSelector: @selector (alloc)] == YES)
...

My confusion is that respondsToSelector: is an instance method (-(BOOL) repondsToSelector:), yet the receiver of the message is the CLASS Square.

[afterDark]

In the documentation, which seems endless and huge, I could find that NSProxy has a class method +(BOOL)respondsToSelector:

but NSObject only has
-(BOOL)respondsToSelector:

Of course, I can and probably have missed something in the documentation. But for now I also do not understand why subclasses of NSObject can use this method.

edit:
I now found Steve's view on this:
http://classroomm.com/objective-c/index.php?topic=6887.msg18791#msg18791
It seems it is not really well known how it works exactly.