Here's a link to the product documentation for what it's worth - the Delete CMDB related entry policy type is described briefly. This policy restricts you to selecting tables commonly related to the CI tables.
As mentioned previously, you can include a condition in the policy's Data filter that checks if the Configuration Item reference field in a related table record is empty, in which case the record will be safe to delete. If in doubt, you can set the "Requires approval" policy option to true in which case approval will be required for the tasks generated for the policy.
Try it out on a sub-production instance where you have some good data to work with i.e. CIs with related table records (e.g. Serial Numbers, Tags, etc) and hope this helps.
