For an introduction on API, first read What is API and High-Frequency Trading (HFT)?
FIX API stands for Financial Information Exchange API. Like all other APIs, it is a set of communication protocols that help different systems communicate. However, FIX API was specifically designed for the transfer of financial data. In other words, it is much more optimized and dedicated to financial trading than other APIs which tend to be more broadly applicable. As such, FIX API yields much more efficient and faster results for traders.
Phemex is one of the very few exchange platforms that actually offers FIX API. This means that our traders using this protocol will benefit from faster and more optimized operations.