The second and third form of the command gives a list of all mail collections that are readable by you. The third form omits the mail collection description.
The first form of this command sets up a current message and a last-read-time for the given mail collection so that when you next log in or issue the @rn command, you will be informed about new mail that has appeared there. Note that this happens automatically whenever you @read messages on a given collection, so if this much is all you care about, you don't need to use this command; just do, e.g.,
@read last on *<collection>
Specifying "with notification" causes you to be added to the immediate-notification list (.mail_notify) for that collection, i.e., whenever new mail arrives there, you will be notified immediately. Specifying "without notification" causes you to be removed from the collection's .mail_notify.
Specifying "before *<other-collection>" causes <collection> to be placed immediately before *<collection> in your @rn listing (which see) and likewise for the "after" clause. By default, new collections are placed at the end of your list. The before/after specification is only useful if you have @mail-option rn_order=fixed set (see "help @mail-options").
@subscribing to a collection for which you already have a current-message/last-read-time has no effect other from possibly changing that collection's .mail_notify and/or reordering your collections for @rn.
You can only @subscribe to collections that are readable by you.
Note that this is entirely different from the Mail Room `subscribe' command
which actually adds you to the .mail_forward list for a given collection/
mailing-list, so that mail sent to the list actually shows up in your own
mail collection.
We're probably going to phase out the Mail Room `subscribe' command...