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The audit host is simply the computer that has the audit scripts installed. This can be the Web Server itself or a second (or third, forth) server with the scripts configured to send results back to the Web Server upon completion of it's scheduled tasks (or cron jobs). Using a separate machine for auditing can reduce the load on the web server which has to process the audit results. Audit Hosts can audit different domains and types of computers within those domains - all separate from one another. For some examples, LINK.
A basic diagram is below.
