The "selectors" used in defining virtual folders are basically

* properties for identifying some group of messages.

They have nothing to do with virtual folders, except that searching is the
most compelling reason to use them.  So, almost all mail clients end up
providing some way to specify such selectors, perhaps through dialogue
boxes.

Whenever you want to specify properties for groups of messages in order to
do something with them, you can use selectors.

- Searching is obvious.  (Leads to virtual folders.)

- deletion (used in vm-avirtual)

- saving (used in vm-avirtual also)

- high-lighting (used in vm-summary-faces)

- hiding (also used in vm-summary-faces)

- movement (e.g., M-n, M-p)

- filtering/archiving/expiration

- spam-selection and deletion

- scoring?




