The plane’s radar cross-section, and the patent paperwork that includes precise technical specifications and outlines. Not incredibly groundbreaking or top secret, but still restricted.

The SU-57’s stealth capabilities were… awful compared to the F-22, F-35, and J-20. Which is disappointing, but expected due to the development hell of its creation, lack of funding or interest beyond propaganda outings, and the fact only 11 have been built over the past 12 years.