SQL server CASE clause when then condition

case 

    when Max_student_allowed > Original_Stu_count     THEN 'A' 

    ELSE 'NA' 

END as RegisterStudent_Status

--------------------

case 

    when Max_student_allowed > Original_Stu_count     THEN 'A' 

    ELSE ' ' 

END as RegisterStudent_Status

reference:-https://www.w3schools.com/sql/sql_case.asp