A Subreport can generate two types of Where clause subqueries:
IN or NOT IN clause (Do Not Join Rows)
Correlated subquery (One or more columns joined to the parent RD)
The closest an RD can come to (1) above is comparing a column to a ValueList or CSV text in the Where clause. This can potentially generate a large amount of ((A = B) OR (A - C) OR...) SQL text. That approach should only be used when it is known the ValueList or CSV test is small.