THE WALL is a FISA "policy that was reduced to writing by DOJ in 1995, and was commonly referenced as
“the wall.”
https://www.fletc.gov/sites/default/files/imported_files/training/programs/legal-division/downloads-articles-and-faqs/research-by-subject/miscellaneous/ForeignIntelligenceSurveillanceAct.pdf
THE WALL policy "the belief was that FISA could not be used as an
investigative tool if the primary purpose of the investigation was, for example, the criminal
prosecution of the target, even if the criminal prosecution would be for the crime of espionage. "