Hello @PRACHIG9

Class Join- If you want to join another class with having one common key or Property 

Example: Class A - Customer Personal Information- Customer ID ,Name,age ..etc

Class B- Customer Salary Information - Customer ID,Salary ,Category 

So here we have common attribute Customer ID we can join these two classes with that common key and can get the salary and his info in the same report definition

Association : This commons into picture where the two classes should join frequently .

Class A-Court- Courtid ,Circuit ID ,Court Name.etc

Class B- Circuit- Circuit ID,Curcuit Name Etc

I this example , A court will have n of circuits under that . So inorder to fetch the Court Info From Circuit  we use common association to call it in all circuit reports to get the court info

Sub Reports : Used in complex requirements   and in the not conditions .Like to calculate percentages and complex calculations.