Every record in a list and every file in a document library automatically has an unique ID within the container (list/document library) it is in. This ID is stored in the property (column): ID
Using the property “ID” is convenient for simple situations when you need an unique property and when the (data of the) business solution is never migrated to another setup.
When the business solution has multiple lists and records are related to each other, using the property “ID” for an unique ID is not a good choice. A major reason for this the migration of data to another location like another environment in the ALM (Application Lifecycle Management) process or another data store like Dataverse.
Using a newly generated GUID for an unique ID when adding a record or file is considered to be a good approach.
The value of the property “ID” is an incremental whole number starting at 1. When the latest item (record/file) is removed, the next record does not get the ID of the removed item but it will get an ID 1 higher than the removed item.
SharePoint only supports the equal (‘=’) operation for delegation on the property “ID”.