When you create the external table, you can specify certain parameters in an access_parameters clause. This clause is optional, as are its individual parameters. For example, you could specify LOGFILE, but not VERSION, or vice versa. The syntax for the access_parameters clause is as follows.
These access parameters are collectively referred to as the opaque_format_spec in the SQL CREATE TABLE...ORGANIZATION EXTERNAL statement.
Oracle Database SQL Language Reference for information about specifying opaque_format_spec when using the SQL CREATE TABLE...ORGANIZATION EXTERNAL statement