What does "fatigued" mean?

[Anusien]

The rules for Extra Effort include this cost modifier (B354):

If you are fatigued, apply a penalty equal to the missing FP.

"Fatigued" is referenced a few other times in the basic rulebook, but I can't figure out exactly what it means. Does it apply any time you have less than full FP, less than 1/3, or some other threshold?

Other places it's mentioned in the basic rulebook:
* The "Awaken" spell defines "very fatigued" as less than 1/3 of basic FP. (B248)
* Sprinting mentions "somewhat fatigued". (B354)
* Interruptions to Enchanting mention "Fatigued" as "missing 1d FP". (B481)

But notably, the thresholds for missing Fatigue Points (B426) don't name the thresholds.