What's the point in the 'gas' parameter passed to function 'signTransaction'?

Question

I am using web3.js v1 in order to sign and send transactions:

async function signAndSend(transaction) {
    let options = {
        to  : transaction._parent._address,
        data: transaction.encodeABI(),
        gas : SOME_GAS_AMOUNT
    };
    let signedTransaction = await web3.eth.accounts.signTransaction(options, PRIVATE_KEY);
    return await web3.eth.sendSignedTransaction(signedTransaction.rawTransaction);
}

In order to compute how many gas units were used, I do:

let gasPrice = new BigNumber(await web3.eth.getGasPrice());
let bgnBalance = new BigNumber(await web3.eth.getBalance(PUBLIC_KEY));
await signAndSend(myTransaction);
let endBalance = new BigNumber(await web3.eth.getBalance(PRIVATE_KEY));
console.log(`Total cost: ${bgnBalance.minus(endBalance).div(gasPrice)} gas units`);

I observe the following:

• When SOME_GAS_AMOUNT is too low, I get base fee exceeds gas limit
• When SOME_GAS_AMOUNT is high enough, I always get the same total cost

My conclusion is, just use a large enough (hard-coded) value.

Am I right?

If yes, then what's the point in this parameter to begin with?

The docs for function signTransaction say:

gas - String: The gas provided by the transaction.

What does "gas provided by the transaction" even mean? I thought that I was the one providing gas for the transaction to take place!

By the way, initially, I used to set the gas parameter as follows:

gas: await transaction.estimateGas({from: PUBLIC_KEY});

However, this occasionally gave me the base fee exceeds gas limit error.

So I figured I was kind of on the wrong track with that one.

In short, any clarifications on the gas parameter would be highly appreciated.

Thank you very much for you help!

UPDATE:

I am not asking about the meaning of the term "gas".

Please refer to my observation (quoting from above):

I observe the following:

• When SOME_GAS_AMOUNT is too low, I get base fee exceeds gas limit
• When SOME_GAS_AMOUNT is high enough, I always get the same total cost

My conclusion is, just use a large enough (hard-coded) value.

Am I right?

If yes, then what's the point in this parameter to begin with?

If no, what exactly should I set this parameter to?

I initially used to set the gas parameter as follows:

gas: await transaction.estimateGas({from: PUBLIC_KEY});

However, this occasionally gave me the base fee exceeds gas limit error.

So I figured I was kind of on the wrong track with that one.