Inferencing in SU is a system of dynamic guidelines and object snaps. As Thom Thom said, you can’t turn it off–it’s like gravity in that regard–but some forms of inferencing are rather bashful, so that not only do you not trip over them all the time, they need to be “encouraged” to make an appearance at all, generally by hovering. One of these is called From Point inferencing, whereby you can extend a dynamic guideline from a point along an axis if you hover over the point first (or as I like to think of it, let the mouse sniff the point). In fact, you can even create two perpendicular From Point inferences simultaneously and snap to their intersection.
Here are some examples of From Point inferencing:
-Gully