An external writer is a started task used to process output. Because the external writer is a started task, it has a userid associated with it. You process output with an external writer by naming the writer on the DD statement (or OUTPUT JCL statement) that defines the output as follows:

//MYOUTPUT  DD  SYSOUT=(A,XTWTR)

For the writer to be able to process that output, the writer's userid must be in an access list that permits the writer's userid to access the SYSOUT data set. The minimum access required for external writers is ALTER. The writer's userid is the userid specified in the started procedure table for the writer task. Alternately, you can define external writers with the trusted attribute in the started procedures table. However, the trusted attribute might be contrary to your installation's policy and allows the external writer access to data you might not want it to process.