There are some situations where a phone needs more than one account to be configured on that phone. These might include
Within this screen you can assign multiple accounts to a master account (i.e. you are editing firstname.lastname@example.org and you want to add email@example.com, 1000 would be the master).
For the same domain simply click the secondary accounts, or if the account you wish to add is on another domain use the second part of the form to do this. If you are trying to add a device from another domain then you may need to give permission on the second domain to allow this action.
Once configured, it normally requires a reboot of the phone and you are away.