I'm guessing that you mean migration. Anyways, if that's the case, then migration is when you send a pokemon from a previous game to a newer game. By doing so, you won't be able to send the sent pokemon back to the previous game.
ie. you have pokemon emerald and you send Latias over to platinum. You won't be able to send latias back to emerald because of changes that could've been done like teaching Latias moves that didn't exist in emerald.
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