What is the BIST MIXED data subscription?

That's pretty easy, it's a light package that includes real-time price data for stocks, futures and 2 indices BIST 100 and BIST 30, however, all symbols in this package are provided without the volume data. 

If you need both real-time price and volume data, you should choose the regular BIST Stocks&Indices package or BIST Futures one.