I am curious as to what is the official allocation for the 685.

At the moment it seems to be any 2 of the MX61 Pulsars and A N Other Pulsar.

Is it meant to be 3 of the MX61s And rotated to even out mileage?

Presumably also what is allocated each must be considered good for a long day out and to cause no operational problems.

Is the 685 viewed as being onerous on the Pulsars? I ask this as other routes, like those up Alnwick an Berwick seem to cause buses to get worn out early.