Dumb question number two -

TPIC ?

Is this some 121 rule ? I've looked in Subpart V, 121.443, 121.543 & 121.545....

as in if I'm flying the leg as FO I can't log PIC per 61.51 ?
I'm assuming, which I hate to do, based on what I've found that TPIC means ACTING PIC ?

Thanks, Jeff