[mick]

DP is a hitter.
FLEX is a fielder for DP only.

DP may play field for any player.
If DP plays field for any player other than FLEX, the other has not left the game. If DP plays field for FLEX, then FLEX has left the game.
(Yes, DP and FLEX may both field with no players removed from the game.)

FLEX may bat only for DP.
If FLEX bats for DP, DP has left the game.

If DP fields for FLEX, Flex is out, but may reenter once.
If FLEX bats for DP, DP is out, but may reenter once.

mick