"is there any situation in flight that would demand Xfer pumps off and Prime on? I can't think of any"
Main tank empty or both Xfer pumps failed, Fuel Pressure caption on one or both engines.
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The MMEL indicates how many of each item is fitted, how many are reqired for dispatch, and the condition of use, ie
CAD power, 2 channels fitted, 1 required for dispatch, VFR flight only. same for VEMD.
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Google EC135 RFM and you'll get all of the emergency captions and actions.