Arc Flash Boundary is an NFPA 70E requirement, the same is true of OSHA 1910.269, NESC and IEEE 1584 also defines this boundary.
NFPA 70E and IEEE 1584 defines this as the point at which an arc flash can transmit energy ≥1.2 cal/cm².
OSHA 1910.269 and NESC require arc flash protection with any exposure ≥2 cal/cm². These two standards do not define "arc flash boundary" but this is still the "danger zone".
The danger at this point is second degree skin burn onset and possible clothing ignition.