Box 1: An additional subnet -
Regional virtual network integration: When you connect to virtual networks in the same region, you must have a dedicated subnet in the virtual network you're integrating with.
Box 2: A VPN gateway -
Gateway-required virtual network integration: When you connect directly to virtual networks in other regions or to a classic virtual network in the same region, you need an Azure Virtual Network gateway created in the target virtual network.
Note: If your app is in an App Service Environment, it's already in a virtual network and doesn't require use of the VNet integration feature to reach resources in the same virtual network.
Reference:
https://docs.microsoft.com/en-us/azure/app-service/overview-vnet-integration
