Yggdrasil

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.

Construction

While there are a few shipyards capable of producing a Titan class ship, the Luna Shipyards at Sol System are the only ones capable of mass-production. Additionally, only planet Earth is capable of properly supplying the contents of a Terraseed.
As such, other shipyards can only produce a poor imitation which will at best support a small base. To terraform an entire planet within a lifetime, rather than a dozen, one will need an actual Terraseed rather than a knockoff. And for that, access to Sol System is mandatory.

Journey

As all Titan vessels, an Yggdrasil can only travel over the Via Solaris. Slow as this is, it can take months or more to reach its destination. During this period, the ship needs a permanent military escort. Not out of fear of pirates, but out of fear of other Corps.
Even losing a single Yggdrasil vessel can bankrupt a smaller Corp. Stealing one would be a great boost and allow for either controlling an additional system, or buying a Leviathan instead. Which means that any suspicious vehicle near an Yggdrasil class will be obliterated by its escorts.