3 inputs determine the right specialty fitting.

A distributor-facing pre-quote checklist. If the customer answers most of these at the first call, the second call is the quote.

1. 01
   Is this an instrumentation / analyzer line or a commercial-vehicle brake circuit?

   The specialty branch is a hard yes/no. Instrumentation / analyzer → double-ferrule SS tube fitting on the B31.3 basis. Brake circuit → DOT. The regulatory category is the answer, not a preference.
2. 02
   What's the tube material and OD, and is it metric or inch?

   Double-ferrule fittings are sized to the tube OD and re-seal on the same ferrules across many re-makes. DOT fittings are built for SAE J844 DOT nylon air-brake tubing. Confirm the OD with a caliper before pulling part numbers.
3. 03
   What's the code basis or certification the connection must meet?

   Instrumentation runs on ASME B31.3 / B31.1. Brake circuits run on 49 CFR Parts 393/571 and FMVSS 121. A non-DOT fitting in a brake circuit is an Out-of-Service violation and unbounded liability if a crash results — there is no industrial substitute.