Tube size gets all the attention, but the pipe or port end is a separate decision with its own standard and its own way of sealing.

A printed pressure rating is not a fact you can compare across catalogs until you know the tube, the wall and the derating behind it.

A named part that will not arrive in time is a paperwork problem before it is a purchasing problem. Handled in order, it costs an afternoon. Handled quietly, it costs the handover.

The unit mismatch is a paperwork problem first, and it is solved on the requisition and on the label.

A material test report is easy to obtain when you ask at the right moment and write it on the purchase order. It is very hard to obtain after the crate has shipped.

Ferrules are spares, and a spare only earns shelf space if the right set reaches the right joint.

When a tubing run has been converted from one fitting brand to another, the paperwork decides whether the line passes.

A male connector has a tube end and a thread end. The tube end is easy. The thread end is where the order goes wrong.

Mixed-unit panels are normal in the field, and the 1/4 inch against 6 mm decision is where a tube list quietly goes wrong.

The union is the part a maintenance crew replaces more often than any other, and the part most likely to be ordered wrong.