The ‘username email address’ is the email address that is associated with an account. It is not necessarily real but it has to be unique for Receipt Bank’s database.
The ‘personalised email address’ is a ‘@receiptbank.me’ address, which is initially automatically generated by Receipt Bank. It is the email address that the user can directly send scanned receipts and invoices to so they can be processed.
Both of those address can be changed at any time via the ‘Edit contact details’ button in the ‘Maintain users’ tab of the ‘Account settings’ menu.