Originally Posted by Check Essential

That's referring to the contractual limit of 6 short calls.
It means the company can't keep you on short call for 3 days straight and only count it as one SC credit. You get 1 credit for each 24 hours.

They can put you on short call for 24 hours a day for 6 days in a row if they want, but that would burn up all 6 credits.