This is what it says in page 7
In order for the LIF migrate to be nondisruptive, the SMB 2.x and 3.0 connections must establish and maintain a durable handle to the open file handle. When a LIF is migrated between nodes, those SMB 2.x and 3.0 clients with durable handle connections have their handles put into a temporary “disconnected” state. In order for the file handles to remain open, the client applications need to reconnect to the “disconnected” file handles
All this tells me is that I cannot guarantee to my managers that there will be no outage in CIFS. You are right, I am not sure how to identify that "durable handle".