Every request from a client is represented in the server as a 'work item,' and the server maintains a pool of available work ...

Every request from a client is represented in the server as a 'work item,' and the server maintains a pool of available work items per CPU to speed processing.  This is the instantaneous number of available work items for this CPU.  A sustained near-zero value indicates the need to increase the MinFreeWorkItems registry value for the Server service.  This value will always be 0 in the SMB1 Blocking Queue instance.