The harness is always the middle layer. You can't point at it in a physics textbook, and you can't point at it in the product spec. It lives in the engineering work between the two — and that's exactly what makes it hard, and valuable.
Step 2 — The same stack across three domains
Three domains, one pattern. The principle is discovered; the harness is engineered; the system is shipped; the output is the value. Swap the domain — the structure is identical.
Step 3 — The key insight
The term "harness" migrated from the stable to the circuit board to the data center because engineers in each field hit the same structural problem: raw capability needs disciplined infrastructure to become reliable output. The word traveled because the challenge is the same.