The P-1 choice lets you factor Mersenne numbers using the P-1 method of factoring.  There is presently no web site which tells you how much P-1 factoring has already been done on exponents.  You can also edit the worktodo.ini file directly.  For example:

 

                Pminus1=751001,1000000,0,0,0

 

The first value is the exponent.  The second value is bound #1.  The third value is bound #2.  The fourth value is 0 for 2^N-1 factoring, 1 for 2^N+1 factoring.  The fifth value is no longer used.