The portal's role in ArcGIS
Portal for ArcGIS plays a central role in organizing and sharing information within your ArcGIS system. The portal provides a user-friendly, searchable repository for your maps and apps. It also helps you create and share new maps and apps.
Some of the rich information in these maps and apps originates from a database within your organization. The GIS content in this database is shared with other devices using web services hosted by ArcGIS for Server. The portal helps map and app creators to find and use these web services. The portal provides an easy window into your GIS content without requiring GIS software training.
Some organizations publish a full suite of web services that can be used to create a fully homegrown app using proprietary data. However, in many cases you might want to supplement your own services with detailed basemaps, place finders, address finders, and other GIS web services from ArcGIS Online. In this scenario you combine your own ArcGIS for Server services with services that are hosted and continuously updated in the Esri cloud.
If your maps and apps don’t have internet access, you can use the Data Appliance for ArcGIS to supply the same basemaps, address finders, and other web services that you would ordinarily find on ArcGIS Online. This type of deployment can be hosted completely behind your organization’s firewall.
If you want to support a combination of internal and external users, you might deploy two portals: one behind your firewall and one that is exposed to the web. This works well when you want full control over the data location and updates, or if your data is not allowed to be hosted offsite. An alternative is to use an organizational subscription to ArcGIS Online for your external-facing apps and services.