The "fear biter" should not be confused with fear aggression. While fear aggression is a proactive "I'm going to get you before you get me or us" behavior, the fear biter is reserved for dogs that only bite in defense and when flight is not an option.
A typical fear biter may "fear bite" when cornered by a perceived threat, when getting nails clipped, or unable to escape any stimulus that elicits a fearful response from the dog.
The true fear biter will have fearful body language and will prefer flight if there is an option.
Pain elicited aggression is also grouped here because of the similar motivation to bite which is the avoidance or cessation of pain.
Both dogs will generally prefer no contact over aggression. The difference is the presence of actual pain in the pain elicited aggression diagnosis vs the perceived anticipation of pain, which may be irrational and based on a phobia, in the fear biter.
A similar comparison can be made of dogs labeled as protective aggressive vs fear aggressive. The difference mainly lies in the rationale of the aggression.