- When you sell or exchange Sketches, you do not retain a copy;
- Sketches disappear after you make the Discovery they were required for;
- If you use the "Sell All Sketches" option at the Observatory, you irreversibly sell all your available Sketches;
- If you make an offer to exchange a Sketch, it becomes unavailable for 3 days. In order to cancel the offer, go to the Market - "Galleons" tab, and select the "Banish" option in the "Loaded Galleons" section.
To unlock special Steam Units, you need to make the Discoveries that require unique Sketches that can be purchased only with Idols. These Sketches can be purchased at the Smuggler's Den, "Idols" section.