각 스킬에는 랭크가 존재 합니다. 
랭크가 올라가면서 지속시간이 늘어 난다던지 좀더 좋은 물건을 발견할 수 있습니다. (각 스킬 설명 참고)

랭크 필요경험치(비우대) 비우대 총계 필요경험치(우대) 우대 총계 
  1       200                      200               100                   100 
  2       800                    1,000               400                   500 
  3      1,800                   2,800               900                 1,400 
  4      3,200                   6,000             1,600                 3,000 
  5      5,000                  11,000             2,500                 5,500 
  6      7,200                  18,200             3,600                 9,100 
  7      9,800                  28,000             4,900                 14,000 
  8      12,800                 40,800             6,400                 20,400 
  9      16,200                 57,000             8,100                 28,500 
 10      20,000                 77,000            10,000                38,500 
 11                                                    12,100                50,600 
 12                                                    14,400                65,000 
 13                                                    16,900                81,900 
 14                                                    19,600               101,500 
 15                                                     22,500              124,000 

위 표와 같이 우대/비우대의 차이는 딱 2배 차이 입니다. 
우대와 비우대스킬은 직업에 따라서 결정이 됩니다. 
각각의 직업에 따라 6~10가지 정도의 우대 스킬을 가집니다

출처:http://dol.mulple.com/
ps:히카루님 홈피에서 동의없이 퍼온것임으로 문제가 된다면 삭제하겠습니다