It means Misc Media. The operator has pressed the MISC button to finalise the sale. This button is used to finish a sale using a payment method other than the ones on the keyboard. E.G you could use it if you accept vouchers for payment. It is like a second cash key with a different label.
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If anyone is looking for a TEC MA-516 cash register manual, go here:
http://the-checkout-tech.com/manuals/cashregisters/TEC2.php
Registerman
June 2010