Similar to how retopo features allow, I would like to be able to have a volume/layer that I can use to affect my active volume/layer. For example if I had a cube or sphere as reference, then my sculpt layer may be influenced/constrained by that layer. On a flat plane intersecting the sphere volume through the center, perhaps the sculpting could build up but not further than the form of the sphere, similar to a boolean operation. Like wise if the plane was above the sphere, I could push the surface of the plane to conform against the sphere volume.
It is useful for sculpting where I can select a part, extract the surface(similar tools but they build a water tight extraction rather than if I select surface data I want, duplicate volume, invert freeze and hide/delete. If I want to use this as similar part to repair part of my mesh elsewhere, it is bit difficult without this sort of feature. Booleans often have trouble. Another use is noisy/damaged surface(scan data), I could repair this alright but have to be careful not to lose the original form too much(or the opposite, I have the proper form that I need the surface to conform to).