Capacity vs Throughput
All too often we equate free time with capacity.
If we do that, we are not building in any slack to deal with interruptions or unexpected work. As a result work piles up and we are constantly behind schedule.
It's important to note that we don't "contain" work we process work. So for us "full" or in other words our maximum limit for delivery should be based on our ability to process work i.e. throughput.
Capacity is an ineffective measure of throughput. And a horrible gauge of what we can do.
For example, on a motorway we are concerned about getting the traffic flowing as quickly as possible. If we focus on maximising capacity then at full capacity we'd have a gridlock and nobody would move.
Our work follow a similar pattern. We need to maximise for throughput when planning and allocating work to others rather than capacity.