You can prevent employees from exceeding their available PTO balance using project assignments and task allocations in CORE. To do so:

1. From the side menu, open the Projects screen.
2. Select your overhead project that is used to track PTO activities and click View Details under the row-action menu.
3. In the detail view, go to General > Assignment tab. 
4. On the Show drop-down, select Activity Items. Click Add Activity Item and select the PTO activity before clicking Add.
5. Next, select Rules on the Show drop-down. Click Add Rule and add the rule 'Prevent time entry after reaching allocated hours.' 
6. From the side menu, open Projects > Resource Allocation.
7. Under the Tasks tab, select Project and choose your overhead project from the Show drop-down.
8. On the grid, select the resource or resource group the PTO rule applies to. Select the PTO activity and enter the units and start and end date for the allocation. You could give 80 units of PTO for the year, for example. You can also select a priority if you want.
9. Click Done to finish.

Employees who attempt to submit hours that exceed their available PTO balance receive the notification 'You have exceeded the allocated hours for this date' and will not be able to submit it. Check CORE Help Center for details on allocating tasks to projects.