those return label, result does nothing. It is the same as doing return result. Learn about comma operator. There's no tuple data type in JS yet.

What you could do instead is the following:

function addFunction(num1, num2, label) {
    let result = parseInt(num1) + parseInt(num2);
    return [label, result];
}

function calculationOfNumbers(x, num1, num2) {
    let label, result;
    if (x == 'add') {
       [label, result] = addFunction(num1, num2, "adding");
    } else if (x == 'subtract') {
       [label, result] = subtractFunction(num1, num2, "subtracting");
    }
    console.log(`This is : ${label} ${result}`);
}

ie., return arrays and use destructuring.

Or even:

function calculationOfNumbers(x, num1, num2) {
    let result;
    if (x == 'add') {
       result = addFunction(num1, num2);
    } else if (x == 'subtract') {
       result = subtractFunction(num1, num2);
    }
    console.log(`This is: ${label} ${result}`);
}

function addFunction(num1, num2) {
    let result = parseInt(num1) + parseInt(num2);
    return label="adding", result;
}

function subtractFunction(num1, num2) {
    let result = parseInt(num2)  - parseInt(num1) ;
    return label="subtracting", result;
}

(I'm not saying this is the best approach tho)

Since you cannot return multiple values with one return statement in JavaScript, one option you could use is to instead return an object like so:

let total = 0;
let num1 = 100;
let num2 = 200;
let text = 'add';
let label = "";

calculationOfNumbers(text, num1, num2);

function calculationOfNumbers(x, num1, num2) {
    let info;

    if (x == 'add') {
        info = addFunction(num1, num2, "adding");
    } else if (x == 'subtract') {
        info = subtractFunction(num1, num2, "subtracting");
    }

    console.log(`This is ${info.label} : ${info.result}`);
}

function addFunction(num1, num2, label) {
    let result = parseInt(num1) + parseInt(num2);
    return { label, result };
}

function subtractFunction(num1, num2, label) {
    let result = parseInt(num2) - parseInt(num1);
    return { label, result };
}

Notice how I assign each function call to the variable info then you can access the required properties with info.label and info.results, respectively.

there are few things needs to be amended in your code

 function calculationOfNumbers(x, num1, num2) {
    if (x == 'add') {
        // function is returning some values, but it was never stored somewhere to be used later
        addFunction(num1, num2, "adding");
    } else if (x == 'subtract') {
      // same situation, function is returning some values, but it was never stored somewhere to be used later
        subtractFunction(num1, num2, "subtracting");
    }
   // you are printing (result) but it was never declared before
    console.log(`This is : ${label}`+result);
}

function addFunction(num1, num2, label) {
  // it's better to use Number(num1) instead of parseInt()
    let result = parseInt(num1) + parseInt(num2);
    // you are returning label inside here, however you didn't amend the global variable result,
// also no need to pass it down to this function, the add function should be returning only the result of the calculation
    return label, result;
}

function subtractFunction(num1, num2, label) {
    let result = parseInt(num2)  - parseInt(num1) ;
    return label, result;
}

So the code after amendment should look like below:

let num1 = 100;
let num2 = 200;
let text = 'add';

calculationOfNumbers(text, num1, num2);

function calculationOfNumbers(x, num1, num2) {
  let label = x;
  let result = 0;

  if (x == 'add') {
    result = addFunction(num1, num2);
  } else if (x == 'subtract') {
    result = subtractFunction(num1, num2);
  }
  console.log(`This is : ${label}` + result);
}

function addFunction(num1, num2) {
  let result = Number(num1) + Number(num2);
  return result;
}

function subtractFunction(num1, num2) {
  let result = Number(num2) - Number(num1);
  return result;
}