where interaction with mobile devices plays a central role (e.g. Making calls, checking messages, displaying content)
This is not restricted to software running on mobile devices (for such, rather use the corresponding tags such as android, ios, or even more device specific iphone-app, ipad-appor windows-phone).
mobile tag is more focused on cases where the software itself is not running on the mobile device itself. Examples include e.g. web-apps using responsive design, or controls to be used from mobile devices (which might also include Bluetooth, IrDa or WiFi).