In Dynamics 365 version 9, the App for Outlook is an app module. However, when initially released, this app module was not configurable.
Microsoft has introduced the ability to filter the entities included in the app module — this means that if you don’t want users to see certain entities in the set regarding dialog, you can remove these entities from the App for Outlook app module. Additional configuration scenarios will be added in the October release.
Official documentation: https://docs.microsoft.com/en-us/dynamics365/customer-engagement/outlook-app/deploy-dynamics-365-app-for-outlook#filter-entities-and-views-that-appear-in-dynamics-365-app-for-outlook