A: Customization and Extension

B: Example: Navigate to FMRental.Extension in the Tree Designer
1. In the Visual Studio, in Solution Explorer, in the FleetManagement Discounts project, expand User Interface > Form Extensions.

The FMRental.Extension element is an extension element that extends the functionality of the FMRental form by adding two new data sources and a new tab control.
2. In Solution Explorer, double-click FMRental.Extension to open the designer.

3. In the designer's search box, type 'e:' as shown in the image below. This filters the current designer to only show nodes that belong to the current extension.

Reference:
https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/dev-itpro/extensibility/customize-model-elements-ex