Rich Communications Service(RCS), is a messaging protocol for Android users that enables read receipts, typing notifications, the ability to share higher-resolution photos and videos and more.
T-Mobile introduced this back in 2015, calling it Advanced Messaging. Many other carriers have moved to do so as well, but the issue arises in carriers having RCS protocols that don’t play well with each other. Carriers moved to resolve this in October of 2019, forming the Cross-Carrier Messaging Initiative(CCMI) to standardize RCS.
Now, the Un-carrier has made a first step in moving towards a universal standar. T-Mobile Android users can now use RCS features with Android users on other carriers through Google Messages, given their phones support RCS Universal Profile 1.0. If you are speaking to other T-Mobile customers, though, there is no need to get an additional app as you can use all RCS features in your phone’s native messaging app.