Table name : SQLnoobhelp 
Id    Name	Category	Quantity
10001 	A 	S 		15
10002 	B 	T 		20
10003 	C 	S 		33
10004 	A 	S 		28
10005 	B 	S 		35
10006	C 	T 		30
10007 	B 	T 		33

Answer:
(SELECT Id, Name, Category, sum(Quantity) as Quantity
FROM SQLnoobhelp
WHERE Name LIKE 'A' and Category LIKE 'S'
GROUP BY Name, Category
ORDER BY Id )

UNION

(SELECT Id, Name, Category, Quantity 
FROM SQLnoobhelp
WHERE Name NOT LIKE 'A' or Category NOT LIKE 'S'
ORDER BY Id)

SELECT  CASE WHEN col1='A' AND col2='S' THEN 1 ELSE Seq END AS Grp,col1,col2,sum(Quantity) AS Total
FROM (SELECT ROW_NUMBER() OVER (PARTITION BY col1,col2 ORDER BY ID) AS Seq,*
FROM Table
)t
GROUP BY CASE WHEN col1='A' AND col2='S' THEN 1 ELSE Seq END,col1,col2