Cargo
The cargo hold is the second grid every ship has, next to its fitting grid. What is in it comes home with you if you extract and mostly stays behind if you do not.
Capacity
The hold is 8 cells wide. Every hull starts with 24 cells and can reach 128. Items in the hold keep their shape and can be turned.
What adds cells: Cargo Bay (+16), Structural Cargohold (+12), a Utility Conduit (+2, scaled by its effectiveness), a MUX cipher (+4), each point in the Utility affinity's cargo option (+1), a cargo modifier on the hull (+2 to +8) and some Regalia bonuses. Some modules take cells away, for example Missile Bay (-6) and Fuel Bay (-4).
If the hold shrinks, whatever was in the lost cells goes to the hub stash when you are in a hub. Anywhere else it is thrown out as loot.
Cargo does not slow a ship down. It does count toward build rating, at a quarter of its rating.
The secure hold
Most hulls have two safety hold cells inside the cargo hold. The Acolyte, Eidolon and Starrk have none. The game marks them Secure Hold: "Items in these slots are extracted with you on death."
The cells sit in fixed places for a given ship, and the places differ from ship to ship.
What survives a loss
Ship loss has the full rules. In short: an item lying entirely on secure cells is kept, an item lying partly on them is kept by chance, and everything else goes to the wreck.