Selection Assistant 
When selecting text on a page, the Selection Assistant will pop up.

Disturbance-Free 
If you don't want the assistant to appear when text is selected, you can turn off automatic pop-up and only invoke it through the shortcut ⌘ + K (Ctrl + K).
Conversation 
Have a conversation with the selected text.
You can set the System Prompt for the current conversation in the dialog box.
