The term mode indicates the action your account or service is able to perform. For example if you account is in test status then you can only perform test transactions but if you account is in Live status then you can perform either test or live transactions.
Test. If your account is in Test status and mode, then you can perform only test transactions, not live transactions. For Website Payments Pro, PayPal cannot determine if you are performing test or live transactions (you can perform both) so it will show "--" as its mode.
Live. If your account is in Live status and mode, then you can perform only Test transactions, not live.