The lowest fare that guarantees* that I sit in the highest cabin offered.
If I'm on a CRJ, I buy Tango.
If I'm on a 787, I buy J, PY, Latitude, or Flex.
If I'm redeeming miles, it's O/I/X depending on the flight.
But I don't sit in Y if there's J, or J if there's F.
*Yes, I know eUpgrades are not guaranteed, but I haven't missed an upgrade since 2014, so I consider them to be pretty much guaranteed for these purposes.