A simple mesh could be cut using the tool: Trim Surface by Curves. In this case:
surface = mesh
curve = spline
Option 1: Direction Orthogonal
In this case the spline cut orthogonally the mesh, it does not matter in which view you are
Option 2: Direction view
In this case the spline cut the part of the mesh covered/hidden by the spline itself, because the Projection is taken normal to the active view. In fact it changes if you are in top view, or Isometric view, etc... If the spline does not cover/hidden any part of the mesh, you cannot cut it (see video)
Option 3: Direction Vector
After choose to trim the mesh using the spline, you can see a red vector: this is the direction in which the orthogonal projection will cut the mesh
Option 4: Direction Normal to surface
The direction of projection is taken normal to the plane of the surface being cut.
How to divide or split a mesh element