For MDES, its dependent on education and also vocation. For e.g. if you sign on as Airforce Engineer and you have diploma, you will start off as ME1. But if you have degree and sign on as Airforce Engineer you start off as ME4.
But if you are signing on as Airforce Engineer (Maintenance) or Airforce Engineer (Flight Engineer), then regardless of education, you will start off as ME1.
ME4 and above is "kind of" considered officer where else ME3 and below is "kind of" like WOSE. Some vocations don't need officers, WOSE will do. Likewise, some vocations don't need ME4 & above, ME3 & below will do.