Help in grouping function desired output

Martin_Kevin 

1. Find 'Active' row as dataset CA;

2. Find ’Max‘ row as dataset CM;

3. Join CA and CM, calculate columns you need.

SELECT CA.RuleID, CA.ActivePeriod Ruleperiod,
  CONVERT(DATE, CA.ActiveDate) RuleDate_of_Active,
  CONVERT(DATE, CM.MaxDate) RuleDate_Max,
  CM.MaxPeriod - CA.ActivePeriod Ruleperiod_Left, 
  DATEDIFF(month, CA.ActiveDate, CM.MaxDate) RuleDate_Left_Round_Month 
FROM
  (
    SELECT RuleID, MAX(Ruleperiod) ActivePeriod, MAX(RuleDate) ActiveDate 
    FROM #Classic 
    WHERE Rulestatus = 'Active' 
    GROUP BY RuleID
  ) CA
  INNER JOIN
  (
    SELECT RuleID, MAX(Ruleperiod) MaxPeriod, MAX(RuleDate) MaxDate  
    FROM #Classic
    GROUP BY RuleID
  ) CM
  ON 
    CA.RuleID=CM.RuleID