Hi @priyal_b,
The method GetMirrorURL() here in the "Others" functions takes a string value as parameter, which is the internal name of the external account configured for your instance. It returns the URL of the mirror page server that you set while setting up the external routing account. If no value is provided here, then the default value set for the instance (in the Deployment Wizard, also available as "NmsServer_MirrorPageUrl" option value) is returned.

I hope that answers the question.
Regards,
Ishan