5 inputs determine the right functional 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
   What does the connection need to do — isolate, check, throttle, swivel, or vent?

   This sets the functional type before anything else. Isolate → ball or hand valve; block backflow → check valve; set cylinder speed → speed controller; swivel with a rotating tool → rotary joint; seal on disconnect → stop fitting. If the connection just joins, a standard push-to-connect fitting is the simpler call.
2. 02
   If it's a shutoff, does the OFF state need to VENT the downstream or just close?

   Vent (operator wants the downstream dead for service) is the 3-way hand valve; a plain lever close is the ball valve. Get this wrong and a device stays pressurized during a repair — a safety miss, not just a spec miss.
3. 03
   If it swivels, what's the rotation speed and operating pressure?

   A rotary joint is rated for both — the bearing and seal have to hold pressure while the joint turns, and the max RPM drops as the tube gets larger. Get the rotation rate and line pressure so the joint is matched to the duty, and use PU tubing at speed — hard tube overloads the bearings.
4. 04
   For a speed controller, is the throttle one-direction (cylinder) or both (line)?

   One direction — a speed controller's integral check meters the stroke and free-flows the return; that's cylinder speed. Both directions equally is a line-balancing job — that's a flow control valve (needle valve), a different part.
5. 05
   What's the tube OD and port thread, and is it metric or inch?

   Functional fittings carry the same push-to-connect tube grip as a standard fitting — 4 mm and 1/4" are not interchangeable. Confirm the OD with a caliper and the port thread (NPT / R-BSPT / G-BSPP) before pulling part numbers.