[ Version ( since = "3.6" ) ]
public void note_vuid_unused (VeeMessageInfoData mi_data, VeeFolder unused_by)
A counter part of note_vuid_used.
Once the unused_by claims the mi_data is not used by it anymore, and neither any other virtual folder is using
it, then the Unmatched folder will have it added.
| this |
a VeeStore |
| mi_data | |
| unused_by |