I think Feargus is referring to the high-level vision documents, representing the major “pillars” or core features of the game.
In my experience, these documents tend to be especially important when you’re designing a new game from scratch. On Obsidian’s earlier projects (before AP), the core gameplay had already been established, and the company‘s job was essentially to build new content within an existing framework. On KOTOR2, they were tweaking the original game’s systems and implementing a new story. The same could be said for NWN2, and it was certainly the case for Mask of the Betrayer.
We had plenty of design documents for those games, but we didn’t have to define our target audience or determine what the player actually *does* and why it’s fun. That requires a more sophisticated process of up-front planning and documentation that was needed for a game like AP - and DS3, as it turned out.