Assertion: The Mandal Commission advised that ‘backward classes’ should be understood to mean ‘backward castes'.
Reason: Many castes, other than the Scheduled Castes, were also treated as low in the caste hierarchy.

Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

The Mandal Commission advised that ‘backward classes’ should be understood to mean ‘backward castes’, since many castes, other than the Scheduled Castes, were also treated as low in the caste hierarchy.