Phones marked with the icon on the right do not support AWS (1700/2100 MHz) UMTS/HSPA+ band. This band is also known as band 4. As of 2016 it's good to have but not necessary to support AWS HSPA+. PCS (1900 MHz) UMTS/HSPA+ support on the other hand is required. 850 MHz UMTS/HSPA+ support is required to access full coverage provided by domestic roaming partners.
Starting from 2015 T-Mobile has been shutting down AWS HSPA+ network in order to use the spectrum for LTE. Indianapolis was the first city to lose AWS HSPA+ in February 2015, followed by Louisville in May and Chicago in June. As of the middle of 2016 AWS HSPA+ network has been shut down in most markets. Almost everywhere it has not been shut down yet T-Mobile still runs 1900 MHz HSPA+. There could be a few places where AWS HSPA+ network is available but PCS HSPA+ is not.
In May 2016 T-Mobile started to sell the first phone that does not support AWS HSPA+, Kyocera DuraForce XD, thus indirectly confirming that AWS HSPA+ support is no longer necessary.