The ALARP clearance certificate details what risk mitigation steps have been taken to manage the UXO risk. The level of clearance will also depend on the available technology, resources, and practices of the day.
The existence of a clearance certificate does not provide a 100 % guarantee that UXO will not be encountered later, but rather that trained staff using the best available technology of the time have been applied to reducing the potential risk from residual items of explosive ordnance.