Neither. It is the actual temperature of your mash tun before you add hot water or grain.
However, it depends on if you add strike water first and then the grain bag or put the grain bag in first and then add the strike water first. I don't do boil it a bag, but if I did, I would do it this way (similar to how I hit my mash in temp with a standard all grain, but not BIAB method).
> Set your mash tun temperature to be exactly what your strike temperature is calculated to be.
> Set your grain temperature for the actual grain temperature.
> Add your strike water to your mash tun at hotter than your strike temperature and stir, waiting for it to drop to your strike
temperature.
(you'll now have a mash tun that is at your strike temperature and strike water that is dead on, which matches
your BS constants)
> Add your grain bag to your mash tun and it should settle at exactly your mash in temperature.
If you were to use your actual cold measured mash tun temperature, then you have to have your mash tun specific heat calculation dead on. For example, if your mash tun is at 70F and you add your strike water, if your specific heat (specific heat = how much or little the mash tun absorbs heat from your strike water) is too low or too high for your particular mash tun, your strike water will cool off too much or too little and it will not settle at your desired strike temperature. My method, overcomes this, because by adding my water hotter (about 6 to 7F) than my strike temperature and stirring for about 5 minutes (usually doesn't take longer than this), I don't care how much or little heat is taken out of the strike water by the mash tun. It adds maybe 5 minutes to your brew day to do this stirring and waiting for the strike water to drop a little, but it guarantees that you hit your strike and mash in temperatures dead on each and every time.
I hope you found this helpful.
PS...Boil In A Bag is actually a misnomer, since the Bag isn't actually boiled. You mash in the bag and at the end of the mash, you lift the bag up and let it hang above the mash tun or boil pot until it stops dripping (squeezing is usually acceptable, since the mesh it usually tight enough in the bag to keep most tannin producing stuff from getting through to your wort). Once it's done dripping, the grain bag is put aside and you commence with your boil. In my humble opinion, a much more accurate term for the procedure would be Mash In A Bag (MIAB).