Achieve 100% of productivity for at least 5 large garages at the same time. As always: It only needs some time. The garages will get a 100% productivity if the employees get a higher level. It can be accelerated by quick traveling between garages.
It is not set as an exhaustion meter ;) It is set after resting time rules in RL, but it is not fully implemented. There are some more regulations there that are not in the game. Actually it seems it does act as a 'meter'. If I travel on the train through the Euro Tunnel it reduces the 'meter' by the time it takes to travel. MEP8.