DP/Flex NFHS Rules

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Here's another way to think about this:

The FLEX's job is to play defense. The DP's job is to play offense. It doesn't mean they cannot perform other "duties as assigned," but that's what they're in the lineup for.

If, at any time, the FLEX is not doing her job (playing defense) or the DP is not doing her job (playing offense), then they have been substituted for and they have left the game and have a re-entry remaining.