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

# Provenance

> Every rate carries a source URL and legal citation. How to read them, and why WITS anchors the non-US non-EU tail.

## The promise

LandedFees never returns an estimated duty rate. Every number on
every response is traceable to a primary source: a tariff schedule
row, a Federal Register notice, a European Commission implementing
regulation, a UK Trade Tariff line, or a WCO Harmonized System code
family. If the engine cannot find the rate, it returns a `data_gap`
compliance flag instead of a fabricated number.

## Where the sources are exposed

Two places on every response:

### 1. `data_as_of`

An object mapping each data source used in the calculation to the
UTC timestamp when that source was last refreshed. Example:

```json theme={null}
{
  "data_as_of": {
    "us_hts": "2026-08-22T14:03:00Z",
    "sec_301": "2026-08-19T11:00:00Z",
    "cbp_fx": "2026-08-21T00:00:00Z"
  }
}
```

Use this to satisfy audit-trail requirements ("the rate applied on
2026-08-23 was current as of the previous business day").

### 2. Compliance flag citations

Every entry in `compliance_flags[]` and `fta_opportunities[]` carries:

* `source`: human-readable authority name (e.g. `USITC HTS 2026 Rev 8`).
* `source_url`: deep link to the underlying document.
* `legal_citation`: the statute or regulation section (e.g. `19 USC 1466(a)`, `HTS 9903.88.15`, `Federal Register 2026-08-19`).

For rate-only responses (`/v1/rates`), the same information ships
under `sources[]` and `data_as_of`.

## The primary-source ladder

| Destination scope            | Primary source                      | Refresh cadence                          |
| ---------------------------- | ----------------------------------- | ---------------------------------------- |
| US                           | USITC HTS                           | Weekly, plus Federal Register push       |
| EU + UK                      | TARIC and UK Trade Tariff           | Daily                                    |
| Everywhere else (\~180 dest) | WCO HS + WITS anchoring             | Monthly, verified per lookup             |
| CBP weekly FX                | 19 CFR 141.62 weekly rate           | Weekly, Wednesday for the following week |
| Section 232 / 301 / 122      | HTS Chapter 99 + USTR               | Business-day polling                     |
| AD/CVD                       | Commerce + ITC final determinations | Business-day polling                     |

## Why WITS is used for the non-US non-EU tail

The US and EU publish tariff schedules in machine-readable form on a
daily-or-better cadence. Most other countries do not. For the
long-tail destinations, LandedFees pulls MFN and preferential rates
from the World Bank's WITS system, which aggregates WTO IDB and CTS
notifications from member countries. WITS is refreshed monthly and
is the most authoritative public source for these destinations.

When you calculate for a WITS-sourced destination, the response
attributes the rate to `wits_wto` and links back to the WITS record.
The rate is still the country's official notified rate; the routing
through WITS is disclosed for transparency.

## Reading a citation

The `legal_citation` field on a Section 301 flag might read:

```
USTR Section 301 List 4A, HTS 9903.88.15, published 2019-08-30
```

Break it down:

* `USTR Section 301 List 4A`: the action.
* `HTS 9903.88.15`: the specific Chapter 99 subheading that
  attaches the ad valorem duty.
* `2019-08-30`: the publication date of the underlying Federal
  Register notice.

Copy this string verbatim into your audit workpapers. It is the
citation that CBP and a broker would recognize.

## Data-gap posture

When a rate cannot be resolved, the response includes:

```json theme={null}
{
  "compliance_flags": [
    {
      "kind": "data_gap",
      "severity": "critical",
      "title": "MFN rate not available for HS 8471.30 in destination XX",
      "detail": "Primary source unavailable. Contact your broker before filing.",
      "source": "WITS",
      "source_url": "https://wits.worldbank.org/..."
    }
  ]
}
```

The calc will still return a `total_landed_cost` value, but the
missing layer's `amount` is `0` with a note. Do not ship a
`critical` `data_gap` response into production without broker
review.