What exactly is tonnage?

[the obelisk (OP)]

What exactly is tonnage in Aurora 4x?  The technical meaning of the term IRL is the cargo carrying capacity of a ship, but obviously that's not what it is in Aurora, so what is it?  I'm familiar with Traveller's use of ton as a measurement of displacement, but it's the volume of a metric ton of liquid hydrogen, which is used as the standard for reasons that are very much particular to the Traveler universe, so that doesn't really get us anywhere.  Is tonnage a measurement of displacement, or a measurement of mass?  There's clearly some kind of abstraction somewhere, but it seems like a pretty even split in terms of mechanics that treat tonnage like displacement and mechanics that treat tonnage like mass, so which is it?

[JacenHan]

Both, and neither. Aurora's tonnage is a rough approximation of both volume and mass, and acts as whichever one is the most appropriate for the situation. You can try to calculate some kind of equivalency between them or a relationship to real life, but I think that it's probably best just to pretend it's roughly the same as ship displacement and let it go.

[Steve Walmsley]

When I created the game, I think I was working on 1 ton = 14 cubic metres, which is the volume of one ton of liquid hydrogen (as per Traveller). Mass and Volume are not treated separately, so a freighter doesn't change tonnage after delivering cargo.

Players can visualize different options though, as it doesn't affect the game.