From pfgwdoc.txt:

Code:

-f[percent][[{Mod_Expr}][{condition}[{condition}...]]]
Modular factoring:
-f{801} uses only primes which are of the form k*801+1
-f{632,-1} uses only primes which are of the form k*632-1
** The {801} and the {632,-1} are the optional {Mod_Expr}
*** NOTE new code added to do both -1 and +1. the format
would be -f{801,+-1} (the +-1 MUST look just like that)
-f{256}{y,8,1) uses only primes which are of the form k*256+1 where
the resultant primes are also of the form j*8+1
-f{256}{n,8,1) uses only primes which are of the form k*256+1 where
the resultant primes are not of the form j*8+1
-f500{256}{y,8,1){y,8,7){n,32,1) uses only primes which are of the
form k*256+1 where the resultant primes are also of the form
j*8+-1 but not j*32+1. There is also a 500% factoring level.
-f{8132}{y,8,1){f,8132} uses only primes which are of the
form k*8132+1 where the resultant primes are also of the form
j*8+1. Also, all factors of 8132 (2,19,107) are checked first.
-f{8132}{y,8,1){p,8133} uses only primes which are of the
form k*8132+1 where the resultant primes are also of the form
j*8+1. Also, ALL primes <= 8133 are checked first.
Note this is also available within the ABC and ABC2 file formats, and
within those formats, expressions can also be used.