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The x/metaprotocol module adds support for encoding and decoding additional fields attached to transactions. extension_options and non_critical_extension_options are optional fields that can be used to attach data to valid transactions. The fields are validated by the blockchain, but they are not used in any way. The fields pass validation if they are provided as empty lists ([ ]) or they use a list of ExtensionData types. The application does not use the attached data but it does ensure that the correct type is provided and that it can be successfully unmarshalled. The attached data will be part of a block.
Txs where extension_options or non_critical_extension_options are populated with a type other than /gaia.metaprotocols.ExtensionData are considered invalid and will be rejected.
Here is an example of a correctly formed non_critical_extension_options field: 
{
  "@type": "/gaia.metaprotocols.ExtensionData", // must be this exact string
  "protocol_id": "some-protocol",
  "protocol_version": "1",
  "data": "<base64 encoded bytes>"
}
Here is an example of a correctly populated non_critical_extension_options on a bank.MsgSend transaction: 
{
  "body": {
    "messages": [
      {
        "@type": "/cosmos.bank.v1beta1.MsgSend",
        "from_address": "cosmos1ehpqg9sj09037uhe56sqktk30asn47asthyr22",
        "to_address": "cosmos1ehpqg9sj09037uhe56sqktk30asn47asthyr22",
        "amount": [
          {
            "denom": "uatom",
            "amount": "100"
          }
        ]
      }
    ],
    "memo": "memo_smaller_than_512_bytes",
    "timeout_height": "0",
    "extension_options": [],
    "non_critical_extension_options": [
      {
        "@type": "/gaia.metaprotocols.ExtensionData",
        "protocol_id": "some-protocol",
        "protocol_version": "1",
        "data": "<base64 encoded bytes>"
      }
    ]
  },
  "auth_info": {
    "signer_infos": [],
    "fee": {
      "amount": [],
      "gas_limit": "200000",
      "payer": "",
      "granter": ""
    },
    "tip": null
  },
  "signatures": []
}