It is plus aliases*. It’s got additional features for them that other providers don’t have though. Like for each label (alias) you can toggle whether to get notifications, mark as unread, screen new senders, and show them in the “aggbox”. The aggbox is like an inbox, but since you don’t ever use your “bare address”, it just shows the labels you want. Your bare address autoresponds with a list of your public addresses.

* It’s technically subaddressing, using either a dash or a plus as a delimiter.