Using Split Face (found under Insert>Modeling Feature>Split Face), you can project a curve onto a face (surface or solid) and then trim or split that face.  But the projection method is different depending on whether you use 2D sketches or 3D sketches as your too entity to do the cutting. A 2D sketch will be projected normal to the plane that the sketch lies on, whereas a 3D sketch will be projected normal to the direction of the target face.

As you can see, using one or the other type of sketch will let you get the desired result depending on what you’re trying to do.

Create fit surfaces by drawing a patch network on the mesh:

As you can see below, there are some cases where the trimmed surfaces will produce a bad result. Specifically, the “Tangent Constraint On Boundary” option, which uses mesh normals to match continuity, sometimes gives a bad result.

(In General, you should avoid drawing patch boundary curves on the high curvature regions of a mesh.)

The following images are the result of a boundary fit rebuilt after optimizing the mesh (Optimize Mesh, then Enhance, thenSubdivide) and adjusting the curvature and size of the mesh.

You may see that the surface quality and continuity between patches are far better than before.

Therefore, the nearly class A quality of surfaces can be simply generated by mesh optimization (It takes only 1~2 min. in XOR)

It is especially indispensable when you match the continuity between patches using the “Tangent Constraint On Boundary” option.

(It’s better to optimize the triangles of meshes to be regular, even if meshes appear to be of good quality. In addition, making sufficiently dense meshes in proportion to the control points of the surface patches can help.)