The key criterion is burst pressure: 391731 covers flexible plastic tubes and hoses with a minimum burst pressure of 27.6 MPa, regardless of reinforcement or fittings. Tubes that do not meet that pressure threshold fall into 391732 (not reinforced, without fittings) or 391733 (not reinforced, with fittings), depending on whether fittings are attached.

Fittings such as joints, elbows, and flanges of plastics have their own dedicated subheading (391740) and are not classified with the pipe or hose they accompany. If a fitting is imported separately, it should be entered under 391740 rather than under the rigid or flexible tube subheadings.