What is standoff height? The standoff height is the distance between the tip of the mixing tube and whatever material your water jet is cutting. If you are not close enough to your material it can change the top edge of your part (see below).
Standoff too low risks breaking the mixing tube which will temporarily shut down your water jet and cost quite a bit to replace. The desired standoff height should be about the thickness of a dime or around 0.100” (2.54 mm).
If the standoff height is too high, you could get a rounding and frosting on the top of your part. This is because the stream is shooting through the air causing the outer layers of the stream to slow and spread out, out making the it wider.
By cutting under water, you can double or triple the standoff height with little change to top edge of your part. However, many operators, understandably, like to see the jet cutting.