What are the "rules" for when it should be used.
You use the asterisk to declare a pointer.
For a Cocoa object, you're always declaring a pointer, so you always use an asterisk. You can't put the object itself into the variable; you always handle a pointer to the object.
For other things, it depends on whether the variable will contain the object (in the C sense) or a pointer to the object-somewhere-else. If the variable should contain the object, then you don't declare it with an asterisk, because you're not putting a pointer in it. If it should contain a pointer, then you do declare it with an asterisk.
You can even have a pointer to a pointer; as you might expect, this involves multiple asterisks. For example, NSRect ** is a pointer to a pointer to an NSRect (which is a structure, not a Cocoa object).
I thought it had something to do with the data type of the variable. (asterisk needed for object data types, not needed for simple data types like int)
Sort of. The asterisk is needed for Cocoa objects because you can only handle pointers to Cocoa objects, never the objects themselves. But the rules for declaration are no different for Cocoa objects; they are exactly the same. You use the asterisk when you want a pointer variable; you don't when you want a non-pointer variable.
The only exception, the only difference for Cocoa objects from the usual rules, is that you are not allowed to declare a variable holding the object itself. That's why you never see a variable holding a Cocoa object instead of a pointer to one: the compiler won't allow it.
However, I have seen object data types such as CGPoint declared without the asterisk as well?
CGPoint is a structure, not a Cocoa object. As such, you can declare a variable that holds a CGPoint and not a pointer to one somewhere else.
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