This is the
🇬🇧 British Tristan da Cunha island flag. The flag is dark blue. The upper left corner is a British flag, and the right is an island emblem. On some platforms the system displays TA.
🇹🇦 is the Emoji of a country/region‘s flag, and its meaning is flag: Tristan da Cunha. The Emoji 🇹🇦 is composed of two regional indicator letters, namely 🇹 and 🇦. The country/region's 2-letter code for Tristan da Cunha is TA, so the letters corresponding to the two region indicator letters are T and A. 🇹🇦 is displayed as a whole flag on most platforms and as a two-letter symbol on a few platforms.