## Common ground
Both options refer to salt system cartridges used in water purification systems.
## Options at a glance
**8421.21 – Generates chlorine from salt**
- **Defining trait:** Produces chlorine through electrolysis of saltwater to disinfect water.
- **Typical examples:** Salt chlorinators for swimming pools, residential water purifiers.
- **Best fit when:** The cartridge’s primary function is active sanitization via chlorine generation.
**8421.99 – Designed for filtering or purifying machinery**
- **Defining trait:** Used as a component in broader filtration or purification systems, not necessarily generating chlorine.
- **Typical examples:** Cartridges in reverse osmosis units, sediment filters, or multi-stage water treatment systems.
- **Best fit when:** The cartridge supports physical or chemical filtration without producing disinfectants.
## Key differences
| Feature | 8421.21 – Generates chlorine from salt | 8421.99 – Designed for filtering/purifying machinery |
|-----------------------|----------------------------------------|------------------------------------------------------|
| Primary function | Electrochemical chlorine production | Filtration or purification support |
| Key technology | Electrolysis | Physical/chemical filtration |
| Output | Active disinfection (chlorine) | Water cleaning without chemical generation |
## How to choose
- Does the cartridge produce chlorine via salt electrolysis? → 8421.21
- Is it used mainly for filtering or purifying water without generating chemicals? → 8421.99
## Overlaps & pitfalls
A cartridge may both generate chlorine and filter impurities, but only one trait should define its classification. If the main purpose is disinfection via chlorine generation, use 8421.21. If it’s primarily a filter or part of a larger purification system without active sanitization, use 8421.99. The defining function (not secondary features) should determine the choice