Apex Sharing Setting

Whenever a record is shared through the apex code, you can specify the apex sharing reason to identify why the record is shared with the user. The advantage of Apex sharing reason is that even though, the owner is a change of the record, the record will be shared with the user.

When working with Apex sharing reasons, note the following:

• Only users with the “Modify All Data” permission can add, edit, or delete sharing that uses an Apex sharing reason.
• Deleting an Apex sharing reason will delete all sharing on the object that uses the reason.
• You can create up to 10 Apex sharing reasons per custom object.
• You can create Apex sharing reasons using the Metadata API.

Reference:https://help.salesforce.com/articleView?id=security_apex_sharing_reasons.htm&type=5