"A 'letter ruling' , also known as a 'private letter ruling' , is a written determination issued to a taxpayer by an associate chief counsel office in response to the taxpayer's written inquiry, filed prior to the filing of returns or reports that are required by the tax laws, about its status for tax purposes or the tax effects of its acts or transactions." (Letter Rulings, IRM § 126.96.36.199.2.3)
"A private letter ruling, or PLR, is a written statement issued to a taxpayer that interprets and applies tax laws to the taxpayer's specific set of facts . . . A PLR is issued in response to a written request submitted by a taxpayer . . ." (IRS Website)
Letter rulings are not published in the Internal Revenue Bulletin.
The following citation resources may be informative.
Below are links to select sources offering private letter rulings.
Below is a pertinent abbreviation.