using IMAP

Does anyone know why, now i have created an IMAP mail account on Mac Mail, that in the usual area at the top of the side bar, i have the Inox, Drafts and Sent, for this IMAP account, and i also have a space down the bottom of the side bar which also lists this account under its own heading, and also has the folders Junk, Trash & Sent.

Which 'sent' do i need to be using? and why?