What it is
Explicitly pre-production
In design, work can plan, draft and propose, but it cannot touch live systems or speak to customers, and everything it produces is clearly marked design-stage.
Why it’s walled off
“Not ready” as a fact
The most common way a half-formed idea does damage is by being treated as finished too early. Phase 0 makes “not ready yet” a structural fact, not a matter of anyone’s memory.
A worked example
A site, still behind the line
A design-stage product can have a whole site and a sharp pitch and still be barred from production — until it clears the gate into Build, it stays behind the line.