"Field Service Advice (FSA) is case specific advice provided to examiners by the Associate Chief Counsel. FSA does not represent a final determination of the Service's position, even in the case for which it was requested." (Requests for Field Service Advice, I.R.M. § 18.104.22.168.1.3) The IRS has phased out FSAs.
Below is a pertinent abbreviation.
Below are links to select sources offering field service advice.