В чем разница между стеком <int>и стеком <string>? может кто-нибудь, пожалуйста, скажите разницу

Question

/*
  Evaluation Of postfix Expression in C++ 
  Input Postfix expression must be in a desired format. 
  Operands must be integers and there should be space in between two operands.
  Only '+'  ,  '-'  , '*' and '/'  operators are expected. 
*/

    #include<iostream>
    #include<stack>
    #include<string>

    using namespace std;

// Function to evaluate Postfix expression and return output

    int EvaluatePostfix(string expression);

// Function to perform an operation and return output. 

    int PerformOperation(char operation, int operand1, int operand2);

// Function to verify whether a character is operator symbol or not. 

    bool IsOperator(char C);

// Function to verify whether a character is numeric digit. 

    bool IsNumericDigit(char C);

    int main() 
    {
        string expression; 
        cout<<"Enter Postfix Expression \n";
        getline(cin,expression);
        int result = EvaluatePostfix(expression);
        cout<<"Output = "<<result<<"\n";
    }

// Function to evaluate Postfix expression and return output

    int EvaluatePostfix(string expression)
    {

    // Declaring a Stack from Standard template library in C++. whats the difference bet stack<int> and stack<string>
    stack<int> S;

    for(int i = 0;i< expression.length();i++) {

        // Scanning each character from left. 
        // If character is a delimitter, move on. 
    if(expression[i] == ' ' || expression[i] == ',') continue; 
        // If character is operator, pop two elements from stack, perform operation and push the result back. 
    else if(IsOperator(expression[i])) {

            // Pop two operands. 
        int operand2 = S.top(); S.pop();
        int operand1 = S.top(); S.pop();

            // Perform operation

        int result = PerformOperation(expression[i], operand1, operand2);

            //Push back result of operation on stack. 
        S.push(result);

    }
    else if(IsNumericDigit(expression[i])){

            // Extract the numeric operand from the string
            // Keep incrementing i as long as you are getting a numeric digit. 
        int operand = 0; 
        while(i<expression.length() && IsNumericDigit(expression[i])) {

                // For a number with more than one digits, as we are scanning from left to right. 
                // Everytime , we get a digit towards right, we can multiply current total in operand by 10 
                // and add the new digit. 
            operand = (operand*10) + (expression[i] - '0'); 
            i++;
        }

            // Finally, you will come out of while loop with i set to a non-numeric character or end of string
            // decrement i because it will be incremented in increment section of loop once again. 
            // We do not want to skip the non-numeric character by incrementing i twice. 
        i--;
            // Push operand on stack. 
        S.push(operand);
    }
}

    // If expression is in correct format, Stack will finally have one element. This will be the output. 
        return S.top();
    }

// Function to verify whether a character is numeric digit. 

    bool IsNumericDigit(char C) 
    {
        if(C >= '0' && C <= '9') return true;
        return false;
    }

// Function to verify whether a character is operator symbol or not. 

    bool IsOperator(char C)
    {
        if(C == '+' || C == '-' || C == '*' || C == '/')
            return true;

        return false;
    }

// Function to perform an operation and return output. 

    int PerformOperation(char operation, int operand1, int operand2)
    {
        if(operation == '+') return operand1 +operand2;
        else if(operation == '-') return operand1 - operand2;
        else if(operation == '*') return operand1 * operand2;
        else if(operation == '/') return operand1 / operand2;

        else cout<<"Unexpected Error \n";
        return -1; 
    }