# ISO 6346 Equipment Category Identifiers (U, J, Z)
> Understand the 4th position equipment category identifier letters (U, J, Z) in container numbers.

- **Canonical URL:** https://stanzaapi.com/tools/container-validator/iso-6346/equipment-category-identifiers
- **Governing Standard:** ISO 6346 / BIC Container Prefix Coding (Bureau International des Conteneurs (BIC) & ISO)
- **Latency:** <1ms Edge
- **Privacy:** In-memory execution only (zero data stored or logged)

## Technical Overview & Governing Rules
Position 4 in any valid 11-character ISO container number must be one of three accredited uppercase letters: U, J, or Z.

## Specification Matrix
| Identifier | Equipment Type | Description |
| --- | --- | --- |
| U | Freight Container | All general cargo, reefer, tank, open-top, and flat-rack containers |
| J | Detachable Equipment | Clip-on generator sets (gensets), refrigeration units, detachable tanks |
| Z | Trailers & Chassis | Intermodal chassis, skeletal trailers, and bogies |

## Standard JSON Response Model
```json
{
  "category": "U",
  "description": "Freight Container",
  "valid": true
}
```

## Frequently Asked Questions & Edge Cases
### Q: Are numbers with category identifiers other than U, J, or Z valid under ISO 6346?
**A:** No, any 4th character other than U, J, or Z is non-compliant with ISO 6346.