I'm new to Project but I'll venture a task as an example of Fixed Work for you:
It's an IT task to develop a suite of programs.
The work estimate is roughly reliable (although it may change if new information comes to light during the task). So I will aim to set and meet the deadline by managing resource units. However, the work is complex and focussed tightly on each single program so there is a limit to how many units can work on a task together effectively. Therefore I cannot make the duration fixed. If we overrun, we have to either deliver late or drop some of the project from the first delivery.
The initial units to be assigned is known up front (although this may also change, for example if other higher priority projects draw resources away, or if work increases and we need to meet the project deadline.)
The duration is something we want Project to track for us, so that we can see if we are meeting the project deadline as things stand.
I'll set the task up as Fixed Work and assign available Units to it, then see the expected Duration.
If the duration is unacceptable, I'll add units as far as I can. The deadline is then set.
If units are withdrawn during the project for more urgent projects, I'll see the duration increase, unless I can add more units.
If a unit meets with a problem or discovers hidden work, we'll increase the Fixed Work accordingly, and see the duration change again.
Hope this helps. I am so new to this I amy be barking up the wrong tree entirely. :)