Originally Posted by cmbdiesel

I'm going to go out on a limb here, and suggest that they arrow (indicating flow direction) is to keep the t-stat from being installed backwards.
Without knowing what vehicle you are installing this on, my guess is that the t-stat inserts in a horizontal position, not vertical? If so, I would further imagine that as long as you see the arrow, then you are OK. Rotating the t-stat will make no difference, flipping it end for end, probably not so good.