Creedmore we posted a bunch on this subject under Palm rest few months ago. However looking at them again  the answer was not simple and concise. Let me give it a try.

With a full Swiss plate Palm Rest and High Cheekpiece Rifle that balances on the Palm rest, most shooters will place the buttplate between the bicep and deltoid muscles.  Idea is the buttplate is not carrying any weight and this get's it away from heartbeats.  However that Bicep positon depends very much on the rifle set up allowing it.  Not all Schuetzen shooters use or used this positon but generaly it's favored with the heavy rifles.

Never seen it placed below the bicep or near the elbow at all.

Boats

advantage to having the butt plate off of the shoulder but not hard out on bicep (pocket in between, favoring bicep side) is it allows the rifle to come across your body more (Lay closer to) creating a closed position and moving the Center of gravity  of the rifle more over your spine. As soon as you get a gun hard into your shoulder especially with a hook the muzzle will point away from your chest (to the right) moving the CG out. This will help your hold. If possible look at olympic smallbore shooters and see how the gun lays.