Target "Virtual"
This is exactly the counterpart of the possible virtual sources in the Main compartment. Choosing a Virtual target here is like placing cables between a Real control element and all corresponding main mappings that use this Virtual control element as source.
Learnable checkbox
If you disable this checkbox, this virtual source will not be learnable via Learn source in the main compartment. This can be useful for rather unimportant control element interactions such as Fader touch that would otherwise make it very hard to learn more important sources such as Fader movement.