The "..." actually represents all the other integers between (for example) 60 and 69.

case 60...69:
return 'D;

is a more concise way for

 case 60:
 case 61:
 case 62:
 case 63:
 case 64:
 case 65:
 case 66:
 case 67:
 case 68:
 case 69:
 return 'D';