You can use both Synchronized Data Extensions and Data Views as data sources when creating Selections in DESelect Segment.
For Data Views, the effect is tremendous, as this is powerful information normally locked to technical users who know SQL. However, with DESelect Segment you can easily get information from Data Views through drag-and-drop.
DESelect Segment support all 23 Data Views available in Salesforce Marketing Cloud. They are available in ‘Available Data Extensions’.
In Data Views you’ll find data like subscribers and Salesforce Marketing Cloud event tracking (email/SMS sends, email opens, link clicks, ...).
Data Views can be easily recognized as their name starts with an underscore.
Related articles: What types of Data Extensions does DESelect Segment support?