An EV charging app is the driver's app that finds a free charger, starts the charge and pays for it. Roaming lets one account use other networks' chargers, with the networks settling the payments between themselves.
Imagine a driver on the Mumbai to Pune expressway whose car has 15 per cent charge left. She opens the app. It shows chargers within 20 kilometres on a map, with the connector type, whether the charger is free or busy, and the price per unit. She chooses a free one and drives there. She plugs in and taps Start in the app. The app sends the request to the charging management software, which tells the charger to begin. When she stops, the app shows the bill and takes the payment through a payment gateway or from her digital wallet.
Drivers need such an app because there are many separate networks, and each had its own app and its own account. The app lets a driver see the status of a charger before driving to it, which matters a great deal when the car has little charge left. It also keeps a history of sessions and bills, which a company can use to reimburse an employee who charges a company car.
Roaming is like a mobile phone roaming on another operator's network. Say the charger belongs to a different network than the one the driver signed up with. Her account still works there. At the end of the month, the two networks settle: the network that supplied the power receives the money from the network that holds her account, minus a small fee.
The app uses the same standards as the rest of the network, such as OCPP between chargers and cloud. Some cars show the same list of chargers on the car's own screen through the infotainment system.