Sent Items
Everything sent from a shared inbox, whichever member sent it.
Last reviewed
Sent Items holds every email sent from this shared inbox's address, whichever member sent it. It's the team's single record of what the address has said, so you can check a colleague's reply without asking them.
A reply appears here once its undo countdown has ended and it has actually gone. If you expected to see something here and it isn't, check for an Outbox in the sidebar: see Fix an email that didn't send.
Sent Items never shows an unread count: nothing in it needs anyone's attention.