Of course, the quality of the manufacturing is Important but besides that the most important distinguishing features are:

  1. Geometry of the receiving port you are using.
  2. Threads call out on the fitting and receiving port, count per inch and total length of threads.
  3. Material the fitting is made out of for purposes of chemical inertness, temperature the fitting will be used for.
  4. Pressure the fitting will be required to hold.
  5. Size of the tubing the fitting will be holding.

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