Plan directives are created automatically by the database and contain statistical data about a re-optimizable SQL statement. This data is used in a future execution of the SQL statement to (hopefully) produce a more optimal execution plan.
SQL directives that have not been used for 53 weeks are purged from the repository.
It should be noted that plan directives are not specifically created for SQL statements, but rather for query expressions used in SQL statements.
Such plan directives are used (for example?) to determine if extended statistics (such as column groups) are missing. (Compare with the dbms_stats preference auto_stat_extension).