Although the use of routes automatically allows traffic to be deployed to your deployment, there are cases where you want to control the routing of packets through a virtual appliance (a network virtual appliance).

To do this, you can create custom routes (so-called user-defined routes) that specify the next hop for packets to a particular subnet, and specify that the packets should be sent to your virtual application instead.

Custom routes are available only for traffic within or between VNets. It is not possible to use custom routes to traffic from the internet. There is only one restriction that is a custom route can only be created using an ARM template, with Azure PowerShell, or with the Azure CLI.