The 6T is the late-year refresh of the OnePlus 6T. XDA-Developers has reported that the company has sold 86% more 6T units compared to those of the 6 on its first day on sale in the United States. This is almost undoubtedly due to OnePlus' new partnership with T-Mobile.
https://www.notebookcheck.net/The-OnePlus-6T-reportedly-out-sold-the-6-by-86-in-US-first-day-sales.360081.0.html