Pipe and tube look the same to the eye but they're sized and made differently. Pipe is sized by 'nominal bore' (NB), which approximates the inside diameter and is intended for fluid flow. A 50 NB pipe is around 60.3 mm outside diameter with a wall thickness chosen from a 'schedule' (Sched 40, Sched 80, etc).
Tube is sized by outside diameter and a chosen wall thickness, with no schedule. A 60.3 mm OD x 3.0 mm wall structural tube is dimensionally similar to a Sched 40 50 NB pipe but they aren't strictly interchangeable. Pipe is rolled to plumbing tolerances and may have a longitudinal seam weld with different inspection requirements.
For gates, fences, and balustrades, you almost always want CHS (structural tube) rather than pipe, but the local steel merchant may stock pipe and call it tube. Check the spec sheet, not just the label.