Problem Description

A simple mesh could be cut using the tool: Trim Surface by Curves. In this case:

surface = mesh

curve = spline

Steps to Resolve

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.

www.youtube.com/watch

See Also

How to divide or split a mesh element

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