What does it mean when a GFCI receptacle is rated 15 amps with a 20 amp feed through?

As I stated in another answer all outlets are really 20 amp rated, but to make them only for 15 amp plugs they only have the two parallel slots and not have the T slot that accepts both 15 and 20 amp plugs, but the terminals behind allow two sets of wires that will connect and "feed thru" to the next outlet.

Thanks TK

Steve