User agents tell the server what browser and operating system the visitor is using to visit the site. You can override the default user agent string to see if there are any significant changes in the way pages are served.
For example, sometimes a user agent for a handheld device will cause the page to behave differently than the default desktop user agent. To test this, run identical audits for each user agent and be sure to change the viewport size to the width of the handheld device.
Best practices are 1) use the default user agent unless there is a suspected issue with a specific one 2) use a different user agent to simulate an audit on mobile devices.
The user agent setting does not change the actual browser used in the audit; it only changes the message sent to the server that identifies the browser. The default is Chrome browser on Linux.
All user agents used by audits and journeys contain the text ObservePoint. This allows analysts to identify traffic in their analytics reports that come from an audit or journey. These have not been registered with any bot registration service.