Click on More options. Here you can either choose a specific DEX or one of Primex's DEX aggregation mechanisms.
In a nutshell:
Selector selects the supported DEX with the best price for your trading pair and order size.
Splitter splits your order across multiple DEXs to provide the best price based on your order size due to the lowest slippage and price impact on each DEX.
Router uses various trading pairs routes to execute your order. Those routes may involve different from your traded assets to get you the best price based on the liquidity available on DEXs. Router is the most advanced mechanism which also combines both Seector and Splitter mechanics.