If it is single PNR with flight originating in US, bagagge allowance of originating carrier will apply for the entire roundtrip ticket at the minimum (Jet can let you have additional allowance if that is standard for Jet in that calss for that segment)

For example if you do JFK-(AA)-LHR-(9W)-BOM-(9W)-DOH-(QR)-JFK in business, your allowance as per USDOT is 2 bags for the entire trip. But Jet will allow 3 bags on BOM-DOH portion as it is standard for them to do so and since they through check to JFK, QR's policy will not come into picture.

Over the years Jet has toyed around with the challenge of limiting domestic bag allowance vs practicality of connecting international pax while at the same time encouraging sirport staff to gouge intl pax for baggage fees wherever they can get away with. There wasa time (several years ago) when the fare was based on IP address location. That does not seem to be case any more.

Also, if originating segment is on Jet, it is very easy to make a booking and see the baggage allowance in the ticket confirmation (As per Jet website, there is no penalty for booking cancelled in 24 hours)