What is true about the MP-BGP extended community attribute?

What is true about the MP-BGP extended community attribute?

A.
If it an AS number form attribute the AS number has to be a registered number.

B.
If it is an AS number form attribute the AS number can be a private AS.

C.
If it is an IP address form of attribute it can be a private address.

D.
If it is an IP address form of attribute it has to use a registered IP address.