# Block transport classes

Radiation diffusivity by block.

File: `data/<namespace>/hbm/rad_diffusivity/<class>.json`.

## Example:

```json
{
  "blocks": [
    "minecraft:water",
    "minecraft:lava"
  ],
  "value": 0.55
}
```

## Fields

| Field | Meaning and default | Range |
| - | - | - |
| `blocks` | Block ID, block tag, or ID list; selects every state of each block. | |
| `value` | Transport coefficient; unmapped blocks use 1.0. | `[0.01, 1.50]` |

## Loading and overlap

Tag membership is resolved when the world opens. Reopen or restart after changing a referenced tag.

When classes overlap, the lowest non-neutral value wins. To change an existing classification, replace the original resource or adjust its block list.

[Radiation-resistant blocks](https://movblock.mov/ntm-next/26.3/wiki/hazards-and-radiation/block-resistance.md) define pocket boundaries by block and state.