If you install the app on the target device, this app is a target app.
It depends on the OS type while you install the app on the target device.
If your target device is an Android, you must install the target app under an APK extension. For example, the file name tts-v1-2.APK.
Please read How to Install the app on Android for more details:
If your target device is an iPhone (iOS), you will install the DEB file from Cydia.
Please read How to install the app on an iOS device for more details: https://iphone.thetruthspy.com/.