what's "id" parameter meaning in JSON-RPC API?


I'm try to interact Ethereum network via JSON-RPC APIs are talked here: https://github.com/ethereum/wiki/wiki/JSON-RPC

I wonder that there is a "id" parameter on all APIs,

for example:
curl example is:
// Request
curl -X POST --data '{"jsonrpc":"2.0","method":"web3_clientVersion","params":[],"id":67}'

the last parameter is "id" : 67, and many API has the same "id",

These 2 APIs have same "id" value 67,

My questions are:
1, What's meaning of this parameter? I haven't got any comments of it.
2, Shall I must use the same value as the example code?
3, Shall I omit this parameter?

I'm highly appreciate for the response, thank you very much in advance.

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