Official letter means formal letter in which no hi hellos..straight forward and no nonsense other than the mentioned subject and it is to the principal boss etc etc
non official letter means informal letter which is reffered to the friends cousins relatives neighbours etc etc. you could write informally hi hellos everything is allowed. you may not need to just talk about the point but you give love affection to the parents brother sister etc etc

hope it helps
plzz mark as best