this setting can be added to the "web.config" file of the home directory of your web server, e.g. "c:\inetpub\wwwroot\arcplan\8\home\web.config". It is used to specify the main domain in multi domain environments. This domain will be prefilled in the login dialog of Engage if set. Syntax to add to the "web.config" file would be <add key="defaultLoginDomain" value="cust.arcplan.com"/> where "cust.arcplan.com" would be the default domain appearing in the Engage login dialog when using Windows Authentication.
For Longview Analytics the document does not exist as there is no Engage Configuration Manager anymore and the setup for Engage is now part of the "Automatic Web Server Configuration", but most of the info you find in the "Engage Quick Start Guide" is still valid for Longview Analytics. Please also have a look into the "Install Guide_Analytics.pdf" and the "Solutions User Guide_Analytics.pdf" you will find here: https://community.arcplan.com/documents/publiclibrary/productdocumentation
Hope this helps, Tasso