Endothermic and exothermic are different categories of reactions and both are also scalar. PMF is exothermic in this analogy. Growth produces more growth by itself, like a raging forest fire. Whereas regular business growth is endothermic: when you stop selling you stop growing.
Endo / Exo have a zero point, across which you discretely jump categories, after which you scale up or down subjectively.
PMF does not have a zero point. If you sell (clasically) "zero to one" item you may or may not have PMF. One nuclear reactor / aircraft carrier is a big deal. One newsletter subscription is a small deal.
