You can select a Windows account to be used when billing archive usage.

The mailbox's primary account. Each mailbox must have at least one Windows user account, called the primary account, associated with it. Select this option to use this account.

This account. If you want to use an account other than the primary account, select this option an specify an account here. Use Browse if you want to select from a list of accounts.

If the mailbox does not have a primary account and you do not want assign a billing account, select The mailbox's primary account.