2026-09-22 Switch line: one four-slot model per product, common supplier clauses at line level

• status: active
• date: 2026-09-22
• sunset: 2027-03-31
• deviates from: docs/documentation-standard.md part 4 ("A product line directory needs only its line spec and brief" — the Switch line directory additionally carries a guide slot), and supersedes 2026-09-14-air-single-file-spec.md
• relatedTask: 20260921-1700-switch-line-document-structure

Context

On 2026-09-14 the Air and Air Hub packages were each collapsed into one self-contained file so that the same figure could no longer drift across a spec, a design appendix, a platform comparison and a supplier guide. Key kept the standard split. One week later the owner found the line unreadable for the opposite reason: the four registers a reader looks for — customer brief, functional description, target specification, development requirements — were in different places for each product, the Key package still restated its chip table in three files under two requirement ID schemes, and the single files mixed internal design chapters into what is handed to a supplier as an RFQ.

Decision

Every Switch product (Air, Key, Air Hub) carries the same four slots and each slot is one register: -brief (customer description), the spec (functional behaviour in chapter 3 plus the target specification; the only file that states a numbered target), -design (internal development input: platform comparison, recommended parts, architecture, budgets) and -guide (supplier development requirements: ID-to-acceptance tracking, verification matrix, factory test, gates, quotation). The clauses common to the three guides — markers, responsibility table, factory flow, D0–D4 gates, quotation format — live once in the line-level bizplan/product/switch/P05-switch-guide.md; product guides cite it and carry only product-specific rows.

Drift is prevented by rule rather than by file count: a design or guide file cites a spec ID or section and never restates a figure; a guide carries no requirement text; the decision frontmatter waiver is removed from both specs so the lint enforces the design slot again.

Consequences

Easier: the same question has the same answer for every product; a supplier receives the spec and guide without internal design chapters; contradictions between registers are caught by the "figures once" rule and the ID-by-ID check recorded in plan 20260921-1701. Harder: four files per product instead of one; a change to a target touches the spec and, through its ID, the guide's tracking row. At the sunset date, review whether the other product lines should adopt the same line-level guide, and whether the documentation standard should name the line-level guide slot explicitly.