Box 26 is for CPP/QPP pensionable earnings. If you didn't pay enough into CPP/QPP, you would have to do so on your tax return, which can cause you to owe money to the CRA.

So if the fact of the matter is that there is no under payment with CPP it should not affect the refund amount you see before entering the amount from your T4 into box 26 correct?

As my colleague previously explained, Box 26 is the amount reported as the total amount of earnings subject to CPP/QPP contributions. Most earnings, such as regular pay, overtime, and allowances are pensionable earnings for the purposes of CPP/QPP.