The "XY Line", was a NKVD term to describe technical espionage. The Soviets found that keeping their technical and non-technical assets seperate improved their intellgence gathering abilities. As a result they created the "XY Line" or technical espionage group in 1941 under the control of Semyon Semyonov (then an MIT student).

Semyonov's targets for XY Line work included engineers at General Dynamics and other aircraft makers, as well as what the Soviets called "Enormoz" the activity the American government named the "Manhattan Project".

Source:  The Haunted Wood: Soviet Espionage in America – The Stalin Era
By Allen Weinstein and Alexander Vassiliev