Basically 11+1(Impact player) can play in every match....so instead of playing 11 at any given point in time they have a piggy bank of 5-6 players off which 1 can become the official 12th guy to play..
So now they declare playing 11 and a pool for that +1 which has batsmen and bowlers ...and use accordingly
You get one extra add on player that's all.
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