It's the temp to maintain the mash at...which often means that your sparge water should be a little hotter...how much hotter depends on your set-up.
If the grain bed/water exceeds 170o F by too much then you can extract harsh/astringent tannins. If the grain bed/water isn't up near that temp then you can leave sugars (or even starches if not fully converted) in the grain bed.
You don't denature all the enzymes when you mash-out @ 170o F. Alpha-amylase doesn't denature until something like 176o F...there is still the potential for some last second conversion to occur in the lauter tun...even @ 170o F. Not that you want to depend on that for conversion...you don't...but it's a little extra benefit of it.
FWIW, I use the ole technology zapap lauter tun and I have to heat my sparge water to 180o to 185o F (some heat is lost before it ever gets to the lauter tun since I'm just ladling it with a saucepot).