> ## 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.

# Tariff stacking

> How Section 232, 301, IEEPA, Section 122, and Chapter 99 overlays combine on a single line item.

## What stacking means

A single import line can attract multiple duty layers at once. The
LandedFees engine resolves each layer against its own statutory
source and returns them as discrete rows in `breakdown[]`. Nothing is
netted, folded, or hidden.

## The overlay families

### MFN (base rate)

The Most-Favored-Nation column-1 rate from the destination country's
tariff schedule. Sourced from USITC HTS (US), TARIC (EU), UK Trade
Tariff (UK), and per-country schedules elsewhere. This is the floor.

### Section 232 (national security)

Applied per **smelter country** on steel and aluminum lines (HTS
Chapters 72, 73, 74, 76). Russia-origin aluminum takes a
200 percent override regardless of exporting country. LandedFees
reads the `smelter_country` field on each line and applies the
correct layer even when the exporter is a passthrough.

### Section 301 (China-focused)

Ad valorem overlays on China-origin lines under HTS 9903.88.xx, from
25 percent (List 3, List 4A) down to 7.5 percent (List 4A reduced).
Some tech goods carry active Section 301 exclusions; the engine
resolves the current exclusion table and returns an `exempted: true`
layer with a citation instead of the duty.

### Section 301-FL (fentanyl)

A separate Section 301 track under HTS 9903.03.02 layered on top of
the base 301 rate. Applies to fentanyl-precursor chemistry lines.
Returned as `SEC_301_FL` when in scope.

### Section 122 (balance-of-payments)

The 10 percent ad valorem overlay under HTS 9903.03.01, currently
active. Applies to a broad slice of US imports across most origins.
Returned as `SEC_122`. When the sunset provision is in effect the
response ships a `section_122` compliance flag.

### Section 338 (retaliatory)

Country-specific retaliatory duties under HTS 9903.90. Currently
scoped to Canadian-origin goods in defined categories. Returned as
`SEC_338_CA`.

### IEEPA overlays

Presidentially-invoked emergency tariff actions land under HTS
Chapter 99 subheadings and are surfaced as their published code.
The engine subscribes to the USITC Chapter 99 change stream so new
IEEPA notices are picked up within one business day of publication.

### AD/CVD

Antidumping and countervailing duties are case-specific. Returned as
`AD_CVD` with the case number in the layer `note` and a `pga` or
`ad_cvd` compliance flag pointing at the ITC or Commerce final
determination.

### FTA preferences

When `usmca_qualifying: true` or `fta_id` is set, the engine tries
the preferential rate first. If the FTA rate beats MFN plus overlays,
`MFN` is returned with `exempted: true` and an `fta_qualified` flag.
When you did NOT claim an FTA but the origin qualifies, an
`fta_opportunity` flag surfaces the potential savings.

## A stacked example

Wireless earbuds, HS `8517.62.00`, origin CN, destination US, customs
value \$6,545. The engine returns:

```json theme={null}
[
  { "code": "MFN",       "amount": 0,       "rate": 0    },
  { "code": "SEC_301",   "amount": 1636.25, "rate": 0.25 },
  { "code": "SEC_122",   "amount": 654.50,  "rate": 0.10 },
  { "code": "MPF",       "amount": 25.67,   "rate": null },
  { "code": "HMF",       "amount": 8.51,    "rate": 0.00125 }
]
```

MFN is 0 percent for this HTS line. Section 301 (List 4A, 25 percent)
lands on the customs value. Section 122 (10 percent) lands on the
same base. MPF and HMF are statutory customs fees, not duties, and
show as separate layers. The engine computes each on its own base
per statute so downstream systems can trace every dollar to a legal
citation.

## Reading the flags

`compliance_flags[]` on the calc response carries any warnings the
engine surfaced while resolving the layers. Common kinds:

* `section_122`: Section 122 sunset scheduled; deadline reminder.
* `ad_cvd`: active AD/CVD case matched; broker review recommended.
* `pga`: Partner Government Agency review required (FDA, EPA, FCC).
* `fta_opportunity`: a lower total exists under a claimable FTA.
* `data_gap`: a layer could not be resolved; do not ship without
  broker review.

Each flag ships `source`, `source_url`, and `legal_citation` so your
audit trail links directly to the underlying notice.