Don't know for sure, but I would guess maybe Safety Officer.
Safety Officers have been added to departments in the last 15 years or so and are responsible for any safety issues at fire scenes. This can be unsafe acts, unsafe building conditions, firefighter accountability, etc. This position has the authority to stop any actions they feel are unsafe. The SO can overrule the incident commander.
BTW, that is the job I have at my FD.
Mike