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It is important to understand the difference between Inventory (also known as items) and cargo (usually referred to as wares). Cargo is in your ship, while inventory is on you. Inventory is listed with the SHIFT I menu under player information. It moves with you when you go (or later teleport!) from ship to ship. Cargo is in your ships storage and can be seen in the ship info menu (Shift Enter for your current ship or INFO on map).
Most Trade and Combat ships have CONTAINER class storage, which are most covers all tradeable goods that are manufactured. Mining ships can have LIQUID or SOLID storage, which covers all materials that can be harvested from space. Trading with ship storage items happens with the TRADE menu when docking, while trading with inventory happens with the trader on a platform.
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