SLA Account Derivation Rule Conditions

The Projects Implementation Guide states on page 3-43 and 3-44, "If you define your own account derivation rules ... you must define a condition ... as follows: ... For adjustment account derivation rules, the source Allow Account Override Flag must equal No"

Please, can anyone explain why this set up is required and how it impacts the outcome of the account derivation rule if set incorrectly to 'Yes' or if it is not included as a condition for the account deriviation rule?

I have not been able to find an explanation in the Subledger Accounting Implementation Guide.

Thank you.