If sendmail is run with a -f command-line argument (-f) and if the F=f delivery agent flag is specified, the A= for this delivery agent will have the two additional arguments -f and $g inserted between its argv[0] and argv[1]. For example, if sendmail is run as:

/usr/lib/sendmail -f jim host!bill

and if the delivery agent for sending to host is defined as:

Muucp, P=/bin/uux, F=fmsDFMhuU, S=13, R=23, A=uux - -r $h!rmail ($u)

the f in F=fmsDFhuU causes the A= of:

A=uux - -r $h@rmail ($u)

to be rewritten as:

A=uux -f $g - -r $h@rmail ($u)

Here, $g is jim from the original command line (but rewritten to be a return address relative to the recipient). The original -f argument jim is first rewritten by the canonify rule set 3, the rule set 1, and then the final rule set 4. The result of those rewrites is placed into $f. The $f sendmail macro is rewritten by the canonify rule set 3, the rule set 1, the S= rule set, and then the final rule set 4, and the result is placed into $g. ($f and $g are described in $g.)

Note that the F=f and the F=r delivery agent flags are very similar and easily confused.