This extension allow split one job from pool to 256 sub jobs, originally created by, full specification on github: NiceHash_CryptoNight_modification_v1.0.txt on user side this is "nicehash" option of pool object.
This is also primary working mode for xmrig-proxy. To make miner works with proxy without additional setup added new item nicehash for extensions field of login reply.


  "id": 1, "jsonrpc": "2.0", "error": null,
  "result": {
    "id": "...",
    "job": {
      "blob": "...", "job_id": "...", "target": "...", "id": "...", "algo": "..."
    "extensions": ["keepalive", "nicehash"],
    "status": "OK"