STI is the abbreviation of English Subaru Tecnia International, which was established in April of 1988. STI belongs to Japan Fuji Heavy Industry Co., Ltd. (FHI for short), and its role is to support Subaru's participation in global automobile races. Among them, the World Rally Championship (WRC) is the main stage of STI. Subaru WRX's racing engines are all assembled and adjusted by STI, and then transported to SWRT, its British racing headquarters.
In other words, he is similar to Mitsubishi Rally Art Company and Nissan Nismo. They mainly do factory car modification.