UV unwrapping: Why do we have to select faces?

Support Materials and Textures
1

The blender documentation here (https://docs.blender.org/manual/en/dev/editors/uv_image/uv_editing/unwrapping/mapping_types.html#unwrap) states that one should “Begin by selecting all faces to be unwrapped in the 3D View.”. However, in my experience, it doesn’t matter if I select once single face, or all the faces of a mesh. The result of the unwrap command is exactly the same.
Why is this?

I’m asking this because i want to know if it’s possible to unwrap only selected faces instead of the whole mesh.
Thanks.

2

Because the UV’s can’t fold open

No. It’s possible to have usable layout for only the selected geometry, but each UV map still has coordinates for everything.

3

It’s possible in the sense that you can select a few faces and unwrap those alone leaving the rest of the unwrapped faces as they were.