The unit number associated with this unit. The unit number can be manually assigned to stack members. Note that the unit number assignment is based on the following factors: - if the unit number is requested here, but another unit already uses that number, the unit changes its configured unit number to the lowest unassigned unit number. - if the unit number is 0, i.e. unassigned, then the unit sets its configured unit number to the lowest unassigned unit number. - if the unit number is configured and no other device uses the unit number, then the unit starts using the configured unit number. - if a unit detects that the maximum number of units already exist, the unit sets its unit number to 0, i.e. unassigned, and stays in the Initialization state. - The max number of stackable units allowed on this chassis is found from the object chNumMaxStackableUnits.