In Service - in maintenace - in training - where does it come from?
It seems to be taken for granted that military ops (especially ships) work on the principal of a third of units are available, a third in maintenance and a third working-up/training
IIRC this first came out the USN Carrier Air Wing system but it seems to be rife everywhere now
Is it based on any hard evidence or is it just a handy way of reducing costs of operations????