It's called a "seed" because, just like all the information needed to create an entire tree is present in encoded instructions within a tree seed, all the information needed to generate the nature and position of every object in a procedurally-generated system is there, in the seed, activated just by feeding that seed number through the stellar forge star system generator. Feed the same seed, and it generates the same star system, each and every time.
This allows for the creation of a truly vast universe without having to store a vast amount of data, either on remote servers or the local computer. Generating a universe this way does of course have its drawbacks:
- Such a universe can never be chaotically dynamic. It can be predictably dynamic - and the ED universe is; the seed generates what the star system would have looked like at "time = 0" (probably 1st January 3300) and then uses Kepler's laws of planetary motion to fast-forward that system map to the current game time. Thus, predictably dynamic. But it can't be chaotically dynamic - such as stars exploding, or planets crashing into each other leaving behind a remnant-planet and a debris cloud. From the stellar forge's perspective, every object it generates has always existed, and always will exist.
- Making changes to the galaxy proves very problematic. You can't simply change the seed, since that would either delete the entire galaxy as it is now known. Changes made to the stellar forge equations would change billions of planets, discovered and undiscovered, simultaneously. This is presumably why "bugs" such as the "Borg-cubes of stars" and the "glowing white giants" have not yet been fixed, since changing them without changing the seed or forge equations would be impossible.