Difference between class join and subreports

Class join is different from subreports. Based on the usage we can use sub reports.
Sub report can be treated as the nested query in SQL, where we get Recordset which we compare with main report.

Class join can be of these types : inner join, left outer join, right outer join.
Class join is to associate two classes with a condition (in a conventional manner).

Note: 

If we already have report which can be used to compare the parent report, then writing the join manually wouldn't be a good idea, as we can reuse exising report as sub report.