A couple things. First that's a 2001 publication/revision. So applicable to the OP's truck, certainly.
Looks like this is now in play, though for newer MY vehicles ...
In the 2010 revision, page 54:
"FAOSC (Rear Fuel Trim) Monitor
As the front UEGO sensor ages and gets exposed to contaminants, it can develop a rich or lean bias in its transfer
function. The rear bias control (also called FAOSC – Fore/Aft Oxygen Sensor Control) system is designed to
compensate for any of these bias shifts (offsets) using the downstream HO2S sensor. The "FAOS" monitor looks
for any bias shifts at the stoichiometric point of the front UEGO sensor lambda curve. If the UEGO has developed a
bias beyond the point for which it can be compensated for, lean (P2096, P2098) or rich (P2097, P2099) fault codes
will be set. "
Which O2 Sensor
(where primary infers the presence of a 'secondary' function - confirmed with a phone call)
BTW - the rest of that thread also has some good info regarding O2's in general ...
I've heard the same thing from other tuners as well.
Odd find: From Page 34 of the 2004 MY manual:
Some 2004 Partial Zero Emission Vehicles (PZEV Focus) will utilize three sets of HO2S sensors. The front
sensors (HO2S11/HO2S21) are the primary fuel control sensors. The next sensors downstream in the exhaust are
utilized to monitor the light-off catalyst (HO2S12/HO2S22). The last sensors downstream in the exhaust
(HO2S13/Ho2S23) are utilized for very long term fuel trim in order to optimize catalyst efficiency (Fore Aft Oxygen
Sensor Control). Ford's first PZEV vehicle uses a 4-cylinder engine so only the Bank 1 DTCs are utilized.
Wow - 6 sensors,