Derby Scheme/Conscription

[MelPack]

I am thoroughly confused by the operation of the age guillotine with the Derby scheme and conscription.

If a single man aged 25/26 in 1916 had completed attestation under the Derby Scheme and was not covered by any exception such as a reserved occupation when would he have been called up?

If the same example had not entered into the Derby scheme when would he have been called up under the conscription system?

Any clarification is welcome.

regards

Mel