Constituent or membership IDs must be unique, so those that are currently in use are not assigned to new records. For example, Constituent ID 10 is not assigned to a new record if that ID is already in use.
If constituent or membership ID numbers become available because the records are deleted, the vacated constituent ID numbers are not automatically assigned to new records. For example, if a record with Constituent ID 10 is merged into a record with Constituent ID 9 and is deleted, Constituent ID 10 is not automatically reassigned to a new record. The next available number is assigned. However, the vacated constituent ID may be assigned manually to a new record.