The AFM for DA40 states in 4A.3.9 CRUISE checklist:
Electrical fuel pump . . . . . . . . . . . . . . . . . . . ON at high altitudes
CAUTION Operation at high altitudes with the electrical fuel pump OFF may cause vapor bubbles, resulting in intermittent low fuel pressure indications, sometimes followed by high fuel flow indications.
I (sort of) understand the reason for needing to compensate for lower ambient pressure with a mechanical fuel pump, but the problem is I can't find a clear reference to what is considered "high altitudes" for this purpose? It has a demonstrated operating limit of FL164, so we're obviously not talking crazy high.
Is it more of a "if you experience these symptoms, try turning on the fuel pump" type of caution? Or is there some specific altitude above which the pump should always be on?