Exception Handling (8e5)

Question

class EmployeeException : Exception
{
public EmployeeException(string idAndPayRate) : base(idAndPayRate + "is invalid")
{
}
}

Can you please explain how did you decide argument is the base ((idAndPayRate + "is invalid")).

using System;
namespace _8e5
{
class EmployeeExceptionDemo
{
static void Main(string[] args)
{
Employee emp1 = CreateEmployee("12", 4);
Employee emp2 = CreateEmployee("34", 20);
Employee emp3 = CreateEmployee("56", 60);
Console.ReadKey();
}

static Employee CreateEmployee(string IDNum, double hourlyWage)
{
Employee emp = null;

try
{
emp = new Employee(IDNum, hourlyWage);
}
catch (EmployeeException ee)
{
Console.WriteLine(ee.Message);
}

return emp;
}
}

class EmployeeException : Exception
{
public EmployeeException(string idAndPayRate) : base(idAndPayRate + " is invalid")//????????????????
{
}
}

class Employee
{
public readonly string IDNum;
public readonly double hourlyWage;

public Employee(string IDNum, double hourlyWage)
{
if (hourlyWage < 6.0 || hourlyWage > 50.0)
throw new EmployeeException(String.Format("ID Number {0}, Pay Rate {1}", IDNum, hourlyWage));
this.IDNum = IDNum;
this.hourlyWage = hourlyWage;
}
}
}
/*
ID Number 12, Pay Rate 4 is invalid
ID Number 56, Pay Rate 60 is invalid
*/