I just reviewed the Columbia 400 POH.

The use "Low Boost as Vapor Suppression as follows"

When operating at high altitudes, 15,000 MSL or above, in hot weather, it
may be necessary to set the vapor suppression switch to ON. Operation of
the vapor suppression will lower engine temperatures and reduce the
chance of formation of vapor in the fuel lines. Operation of the vapor
suppression may be required at lower altitudes when the ambient
temperature is significantly above normal. Vapor suppression must be
turned on if TIT is rising above 1460ºF at full power and the mixture is
set to full rich (at any altitude). Vapor suppression may be turned off
below 18,000 MSL if power has been reduced below 85% and engine
temperatures have stabilized.

The checklists call for "Vapor Suppression above 18000"
They also turn it on for fuel tank changeovers.


During take Off they ARM the High Boost. The High Boost will actually engage
and latch on if the fuel pressure drops below 5.5 PSI. (This is ARMED
after setting the mixture to full rich). This is left on thru climb and
disabled after reaching cruise. It is always off during a descent.

They indicate that the LATCHING relay will also disable the low boost if
the high boost is called for (Low fuel pressure).