The Z of your till is the document that summarises your cash receipts over a given period. As a general rule, this document is printed out from your till at the end of each day to show the day's sales.
Once this document has been printed out, you will no longer be able to modify it. This is because the cash register system has to record all the original data relating to payments, keep it and make it unalterable.
You should only print out the Z once a day, and preferably at the end of the day, when you do the usual checks on cash receipts.
If you want to know the amount of cash received during the day, don't print out a cash desk Z which closes the day's transactions. Make an X cash register which will have the advantage of giving you almost the same information as a Z cash register but will not reset your data for the day to zero.
It is useful for carrying out checks during the day and before closing the till.