Most basic cash registers will come with a set of keys.
One key will be used to lock the cash drawer, it cannot be used to open the drawer. It is just used to lock the drawer for extra security. When the drawer is locked you will be unable to fire it open using the emergency release.
You should also receive an operator key, this can only be used to access reg mode on the control lock and to make a sale.
The manager key will be required if you wish to take any reports, put through a refund or make any program changes.
With a more advanced cash register you may also receive an S key, this can be used to access a hidden service mode.
You can use these keys to restrict clerk access but however, if for example you leave a clerk with just an operator key and they have a problem with the cash register they may be unable to fix it.
There are some much better options available and we would not recommend using these keys to restrict clerk access. If you do use these keys to restrict access then it is advisable to ensure that you always have someone on site with access to the drawer key, operator key, manager key and service key.