The flavour unit (FU) is a dimensionless number calculated by the concentration of the flavour compound divided by the sensory threshold. If a compound is present at less than one flavour unit it is below the sensory threshold, if it is present at one to two FUs then it is detectable and if it is present at greater than two FUs it is likely to have a major effect on flavour.
Paul Hughes was one of my lecturers at Heriot-Watt so I have come across flavour units before, though they're certainly not something widedly used in the industry. They sound very useful for describing beer flavour but I suspect that using them involves a lot more work than, for example, simply listing the lab analysis (or more likely calculated) International Bitterness Units (IBU) even though IBUs are a very poor flavour indicator.
Beer flavour is extremely complicated but the provide a handy table of the compounds which sheds some light on it.
Number of flavour units
Greater than two (substantially affect beer flavour)
Niche and defective beers*
Iron, copper ions
0.5-2 FUs (cause small flavour changes)
Fatty acids (C6-C10)
Other hop compounds
Niche and defective beers
0.1-0.5 FUs (changes at these levels do not affect beer flavour)
The complexity of beer is highlighted by the note "compounds present at less than 0.1 FU are considered to account for less than 30% of total beer flavour", which to me still sounds a reasonable about for compounds you're not even close to tasting individually.
The authors conclude the chapter:
"For many beers, bitterness, alcohol and carbon dioxide dominate the sensory profile of beer, and are characters readily perceived by the consumers. Other characters, such as hoppy, fruity/estery, dimethyl sulphide and sulphury, contribute to brand differentiation. Finally, compounds such as diacetyl and aliphatic aldehydes are often indicative of defective beers."
The flavour unit ranking reminds me of George Fix's primary, secondary and tertiary effects, though with a lot more specificity. And though I fully approve of obsessing about all the details when brewing, remember some are more important than others.
* Sounds like a synonym for craft beer, doesn't it?