You can specify whether a unit is in use or not in use. Open a unit and click on Edit, then simply choose between In use and Not in use.
A unit that is marked as Not in use will have a red marker in front of it in the list of units when you want to carry out a check in app.
In the unit list in the Admin Panel the status of each unit is shown by either a green or red marker. You can also choose to only display units that are in use or not in use.