Custom URL scheme is the mechanism through which third party applications can interact with each other. Suppose we want an interaction between our app and the default email application in the device. Apps that support custom URL schemes can use the schemes to communicate with other applications and also initiate specific tasks. For example, an app can open from default email app in IOS devices using custom URL.
To communicate with an app using a custom URL, we need to register the custom URL scheme in our appname-info.plist file. Apple by default supports for the http, mailto, tel, and SMS URL schemes. So if our URL scheme matches with the scheme mentioned by Apple then the Apple provided application will launch instead of our application. Therefore we need to be very careful before declaring the scheme.