A subquery is either correlated or non correlated.
A correlated subquery references a column in a table that is listed in the from list of the outer query. If the subquery is correlated, showplan includes the message “Correlated Subquery.”
A non correlated subquery can be evaluated independently of the outer query. Non correlated subqueries are sometimes materialized, so their showplan output does not include the normal subquery showplan messages.