CTS (Compatibility Test Suite): CTS is a collection of tests used to verify the core Android functionality and compatibility of devices with the Android Open Source Project (AOSP) specifications. It ensures that an Android device conforms to Google’s compatibility standards, which is essential for manufacturers aiming to use Android as a core operating system. Passing CTS is necessary for device compatibility but does not cover Google Mobile Services (GMS) requirements.

GTS (GMS Test Suite): GTS is specifically designed to test compliance with Google Mobile Services (GMS) requirements. This suite of tests validates that a device meets Google’s standards for pre-installed apps and services like Google Play, Maps, and Gmail. Passing the GTS is necessary for a device to be licensed to include Google’s proprietary applications and services.

more in www.android-test.com. a resource platform dedicated to testing and certification for various Google Android devices.