Check unlock codes allow families the ability to register for a program without paying the registration or other fees up front. These codes would need to be given to parents in order for them to be able to use a code during registration.
From the Setup menu, select System Config, then select the registration tile.

Then select Registration-

Enter the unlock code you want to give to parents to pay by check, then scroll to the bottom of the page and press Save.

You can also specify a check unlock code by specific category. Go to Setup > Classroom

Select the name of a Category

Change Opt Out of Unlock Code for Checks to No and then enter the text/numbers that parents will use to unlock the check option during registration.

Scroll to the bottom of the page and Save.
To update a Check Unlock code go to either area and type over the text that is in the check unlock field, then Save.