The decision model

For a state, DefaultNexusEngine::evaluate() resolves a NexusStatus in this precedence:

1. Registered — the seller already holds a registration there (obligation handled).
2. Triggered (physical) — asserted physical presence establishes nexus on its own, whatever the sales.
3. Not applicable — the state levies no general sales tax (DE, MT, NH, OR). There is no threshold to cross and nothing to register for.
4. Unknown — the engine cannot evaluate. See below; this is the important one.
5. Triggered (economic) — cumulative activity crosses the threshold (combinator-aware: sales-only, sales-or-transactions, sales-and-transactions).
6. Approaching — activity is within the configured band (default 80%) of the threshold. A watch signal, not an obligation.
7. Below — under the band.

"I don't know" is not "you're fine"

Unknown exists because the two are different answers and must not share a case. A state resolves to Unknown when:

• the threshold cannot be resolved — the dataset is fetched over the network, so a firewalled or misconfigured install resolves null for every state;
• the ledger cannot answer — SalesLedger::activityFor() returned null, meaning unbound, unreachable, not yet populated, or — for a host serving several sellers — asked without a subject to answer for. To assert that nothing was sold, a ledger returns new SellerActivity(0, 0), which is what a real one does (a SUM over no rows is 0);
• the totals were counted against a different question — the host declared a basis or period on its activity that disagrees with what the state measures. Florida measures taxable sales over the previous calendar year; comparing gross calendar-YTD totals to its $100,000 is not an approximation of the answer, it is a different number.

Reporting Below in any of those cases reads as compliance. A seller could cross every threshold in the country and see a clean board.

Acting on the result

Triggered and Unknown both return needsAction() === true. Unknown does not need a registration — it needs an operator to find out why the engine could not answer. Treating it as inert is what let the gap stay invisible.

Each NexusEvaluation carries the threshold, activity, a progress ratio (0–1+ toward crossing), physicalPresence, a human-readable reason, and caveats.

Caveats

caveats records what the verdict rests on that the engine could not check — chiefly a host that fed totals without declaring which sales it counted or over what window, or a state the dataset carries no basis for. A verdict with caveats is still the best answer available, but it is not one the engine verified end to end:

$e = $engine->evaluate($state);

if ($e->isQualified()) {
    // "Register" — but which sales that rests on was never verified.
    foreach ($e->caveats as $caveat) { … }
}

Reporting

A NexusReport buckets a set of evaluations into triggered(), approaching(), registered(), unknown() and notApplicable().

Prefer needingAction() for a dashboard's primary list. It rolls up triggered and unknown in one call, so a consumer cannot ship a reassuringly empty board by rendering three buckets and forgetting the fourth.

$report = $engine->report($states);

$report->needingAction();   // crossed obligations AND states it could not evaluate
$report->notApplicable();   // DE, MT, NH, OR — nothing to comply with