A special type of Titan starship, the Yggdrasil class are built to contain Terraforming Seeds and safely carry their contents to other star systems. Here, the ship will be able to slowly unload its cargo and let its owners terraform enough of a planet to make it a safe place to live. To keep the precious cargo of these ships in good shape requires detailed climate control, cryogenic areas, even significant artificial gravity. This is why only Titan class ships can be properly outfitted to maintain a terraseed. Additionally, the structure required is designed in such a way that it's not possible to retrofit a normal Titan or Leviathan into an Yggdrasil, or vice versa. Worse, the contents of an Yggdrasil vessel are as expensive as the vessel itself, while getting all of the contents out means the ship has to trigger irrevocable dismantling procedures. So forget about getting a deposit, or bringing the ship back to refill it. They're one-time use, which is why they're one of the biggest investments involved for Corps claiming new systems.