Hi igor, just one more note:

> I was able to delete a trim curve on the backside of the
> cylinder,

Yup, that was because it was an interior "hole" boundary, so just that one edge defined an entire trim loop.


> but curve in front got splitted by seam edge of the cylinder and no
> matter what I try it refuses to be deleted))

That's because you were probably not selecting the entire trim boundary, the seam is basically like one of the "outermost" natural edges of the surface, when something intersects it, it becomes part of a longer loop for the exterior trim boundary rather than an interior hole trim.

To remove a boundary you need to select all of its edges (and they must not be attached to any other surface), so in this case with the piece that intersects the seam you must select the seam edges, and the top/bottom edges as well since they are all part of that trim boundary.

- Michael