Its sounds like you are trying to create a surface region likefrom Solid works??
If this is so then you can project a curve onto that surface and then make a copy of the surface and trim it back to the projected curve.
Another way is to instead of projecting your sketch you can extrude it as a surface and then click on the cut option in the extrude dashboard and click the surface that you want to make the region on, you see a flip arrow which if you click it will switch from one sode to another, but you can also keep both side, indicated by 2 arrows in opposing directions, this gives you 2 seperate surfaces. It all depends on what you are trying to achieve, a few images or a bit more detail might aid us to help you better.
Paddy
Edited by: mcgowanp