header vs. sub-header

Perhaps, another would be the type of P&ID we are looking at may give us a clue.
i.e. 1. Distribution P&IDs will have Headers / Sub-headers mostly.
2. Process / Other Utility P&IDs will have Branches off 'Headers & Sub-headers'

Mostly, as soon as there is Bore change / Design Break, line number tends to change.

I'd call the line running on the main arterial rack as my header and the distribution headers off it, that go to individual areas as sub-headers. At least the piping guys with me would understand it !