I have an Editable Mesh with Unwrap UVW modifier
In the Edit window of the modifier, in Face sub-object mode
it is not possible to select a subset of faces … either all faces or none are selectable
If I create another Editable Mesh (eg. from a simple Plane) then I can select single faces …
As a workaround I broke the Editable Mesh into Elements, then applied the UVW modifier, then applied another Editable Mesh modifier and welded the vertices back together, this worked OK
Any idea why one Editable Mesh would disallow face selection?