According to the site, the first step is Manufacturer Development. At this stage, T-Mobile and the manufacturer of the device both agree to support a software update. After that comes Testing during which the device goes through the T-Mobile certification process. The last stage, Completed, means that the update has received the approval to launch, and the software is available for T-Mobile to disseminate it.
The T-Mobile Galaxy S6 and Galaxy S6 edge are still going through the necessary testing, and while we don't know how much longer until Android 6.0 is running on the handsets, T-Mobile subscribers with either model are now a step closer to receiving the update. Android 6.0 includes Doze, which puts your phone into a deep sleep state to improve battery life. Google Now on Tap provides contextual search results based on the content of your screen. And app permissions can be approved on a feature-by-feature basis.
source: T-Mobile via TmoNews