The US recently sanctioned China for buying the S-400 and Su-35 fighter jets from Russia. A waiver for India for buying the same weapons system will send a positive message to India about the US recognising India’s security needs. However, there are warnings on interoperability with US and other Western systems. India has had Russian air defence systems for decades, and they are integrated with other western systems like fighter jets and radars.
Some reports indicate that the US has precluded any potential F-35 stealth fighter jets sale to India. This is likely a coercion tactic to stop India from buying the S-400, and also to push US air defence systems, like the PAC-3, in India. The US will not refuse a potential 200 fifth generation fighter jet requirement of India.