Designated Player & Flex Player Defensive Roles: Clarification

Question
The player designated on the lineup card as the DP entered the game to play defense for a different player than who he was hitting for.  We were under the impression that the DP can only go in for the person he is hitting for which that player is listed as the Flex.  If the DP is hitting for the pitcher (Flex), than the only defensive position he can play is pitcher.  

Answer
Hi Chuck,

The dp can play defense for any player.  
Both the dp and flex can play defense at the same time.  
They (the dp and flex)cannot be in the offense at the same time.  
If the dp is playing defense for someone other than the flex that person would still bat and are not considered to have left the game.

Mark