If there is more than one register set up for a location, then an employee will have to choose which register they are using to complete sales as soon as they select the sales feature. For information on setting up registers, see the documentation here.
You can always see the register you're working on in the top header. You can change registers at any time if the wrong one is selected. In the header of the sales page, click the [Change Register] to the right of your currently selected register.
You’ll be brought back to the selection screen where you can pick the desired register.