The thematic layers in your GIS can come from many sources. They can be base layers, such as street maps and imagery from ArcGIS online. They can be foundational layers stored in a geodatabase, such as a parcel or water utilities layer complete with associated rules and behavior. And they can be layers you've pulled into the map from external business systems, such as customer information or sales reports.
ArcGIS allows you to connect to data in external databases, bring that data into your map and work with it alongside your own. ArcGIS can work with many different data formats such as shapefiles, text, spreadsheets or other tabular data, numerous kinds of imagery, etc…. Once pulled into a map, that data can be leveraged for viewing and analysis.
In the following video the value analysis dashboard is used as an example of how you can work with external data.
In the video you can see how tabular data from the local government's tax and deeds systems is pulled into a map and integrated with the other layers. By combining it with the spatial attributes of the other layers, that tabular data becomes visible on a map and can be used to generate reporting layers for additional analysis and visualization.
Data in external business systems, such as the tax information from the local government's databases, can be a valuable asset. ArcGIS gives you a platform to explore the strength of that data and easily work with external data in conjunction with your own.