Why nuclear use is categorically different from thermobaric attack

Why would Russian use of a nuclear weapon in Ukraine produce a different response from thermobaric weapons, even if the nuclear yield were small?

Fallout could reach NATO territory, and the political fact of nuclear use would trigger a much broader escalation and isolation response regardless of yield.

Thermobaric weapons can cause immense destruction, but they do not create the radioactive fallout that makes nuclear use cross borders physically as well as politically. A strike in Ukraine could send contamination into Poland and turn the attack into a direct problem for a NATO member. Nuclear use could also push Russia into a more threatening category of international isolation and strain its relationship with China. Public and political reactions would focus on the fact that a nuclear weapon had been used, regardless of whether it was small, specialized, or lower yield. The threshold is therefore categorical: technical efforts to describe a limited nuclear use would not remove its escalatory meaning.