How to select part of a mesh by using a helper plane? I would like to select part of a mesh using a plane as a &quot;boundary indicator&quot;.
In the screenshot below that would be the mesh part above the plane:

I can get the desired selection by using Bisect tool (Bisect, hide, select a vertex, select linked, unhide):

however, this is visually not as convenient as using a plane (this is only one step in a longer workflow which has to be simplified). Beside that, Bisect cuts trough the whole object, what is inconvenient (notice the bisect-line cutting another part of the mesh in lower right).
I know I can use Boolean modifier to get only the part above the plane, but I do not want to cut it off - I just want to select it for editing.
The &quot;transition line&quot; does not have to be straight (i.e. adding vertices and edges), it can use existing edges (resulting in a jagged &quot;transition line&quot;).
Edit:
What I meant is to use the plane in a similar way it would be used in Boolean modifer, just not to &quot;cut off&quot; but to select that part of the mesh.
The goal is to automate the process, e.g. operator positions the plane and clicks on a button (to run a script that does the rest; I can write the script once I know the steps).
1. Lasso Select:

You can use Lasso Select with Ctrl + LMB.



Here a related Post for types of selection.

Limit selection to visible should be disabled to select all vertices in the Lasso area.


Image by iKlsR♦ answer





2. Object as Camera:

This solution involves a 2nd Object which is perpendecular to the Plane and use it as a Camera. That makes it easier for a Border Selection and the 3D Cursor is used for the orientation.




Position the Plane at the desired Location.
Dublicate the Plane with Shift + D in Object Mode
Rotate the duplicated Plane by 90° around the local X-axis. R + 2xX + 90
Translate the rotated Plane along the local Z-axis with G + 2xZ.



Select the first Plane and press Shift + S > Cursor to Selected.
Select the second Plane and press Ctrl + Numpad 0 to make the Plane the active Camera.
Select your Object, toggle to Edit Mode and Border Select B the desired Vertices. The 3D Cursor helps where to stop with the selection.


Vertices selection successfully