Yes, It's a well noted irony that the subject of the document has no definition. the problem is that it is very hard to define, or diagnose. Often, the crew member themself may not know tha they are fatigued. The onset of fatigue is very insidious and it takes either an incident or a friend/colleague to point it out.
Also, fatigue affects diffrent people to varying levels. Persoanlly, I found that as I got older, my circadian rythym became more entrenched and I was finding it harder to get effective rest. this is one of the factors which made me decide to resign from a long haul job and start flying short haul. I still get fatigued (I've just come off a 9.5 hour duty ending at 1.30am and am having my wind down beer while PPruNing) and am pretty tired. But I'm not suffering acute fatigue on top of underlying chronic fatigue which I often suffered on long haul operations and which in my experience, is the most dangerous set of circumstances.
As for the reporting of fatigue, it's one area of flight safety which is not well served. The only effective avenue is one of the various confidential schemes which exist around the industry. As I metioned, 'fatigue' is the one word that managers do not want to hear. Perhaps because it's so prevalent. And perhaps now is the time for someone to bite the bullet and introduce a scheme. But unfortunately, as flying becomes safer with the introduction of systems such as TAWS and ACAS, pilot error statistics are proportionally on the rise. And fatigue related incidents are featuring more. So there is some reporting going on!
I'm glad my post may be of some help.