A question about Forward ACKed packets

Hi,

I'm trying to understand the concept of Forward Acknowledgements
(FACK). What confuses me most is the variable fackets_out, which,
according to the source code, is equal to the "FACK'd packets".

Isn't by definition only a single packet "the most Forward ACKed
packet"...? If not, what are "FACK'd packets"? Is fackets_out the
number of "gaps" between SACKed data?

Greetings,
Dominik