> ## Documentation Index
> Fetch the complete documentation index at: https://www.docs.forgeshipyard.com/llms.txt
> Use this file to discover all available pages before exploring further.

# L4 · Rules & Applicability Engine

> Decides which rules apply, what evidence is missing, and what conflicts exist.

export const Supersession = () => <Tooltip tip="Replacing a prior decision or rule version with a new one while keeping the link back to what was replaced.">supersession</Tooltip>;

<Note>
  **In plain terms:** L4 answers "which rules apply to this design, what supports
  that, what is missing, and what conflicts?" It is the layer that automates the
  regulatory gap analysis engineers do by hand today.
</Note>

## What it does

L4 applies ABS, USCG, IACS, IMO, owner, and yard rules with citations and gap
analysis. For a given design it answers:

* Which rules apply?
* What evidence supports that applicability claim?
* What is missing?
* What conflicts exist?

## The problem it replaces

Today, when a U.S. yard adapts a foreign design, engineers manually alter blueprints
to comply with USCG and ABS rules, which are materially stricter than international
class rules. Small differences cascade. For example, international rules may allow a
700-millimeter passageway while the USCG mandates 900. The added width pushes
bulkheads outward and clashes with pre-planned electrical and piping systems.
Because procurement runs in parallel, these problems often surface after steel is
already bought.

L4 surfaces those conflicts as the design is built, not after procurement has
committed. CAD platforms identify geometric clashes once they appear; L4 extends
this by predicting cascade effects *before* they appear, and by connecting material
substitutions to their effect on stability before a purchase order is signed.

## Handling rule changes

When a new rule version arrives, L4 emits a rule supersession event that identifies
the prior version being superseded. That event triggers the re-evaluation queue
described in [When the Rules Change](/concepts/when-rules-change). This is where
<Supersession /> begins.

## What L4 outputs

* `applicability_result`
* `compliance_gap`
* `evidence_match`
* `rule_supersession_event`

## Quality targets

L4 holds itself to measurable bars before any production use:

* Rule applicability **precision** better than 90% (false positives create review
  burden).
* Rule applicability **recall** better than 85% (missed rules can be dangerous).
* Missing-evidence detection **recall** better than 90%.