The advanced settings are available through configuration files on both the host and the client, and allows for some settings not shown via the application itself. You can access the configuration file going into Parsec's settings, scrolling to the bottom and clicking "edit the configuration file directly". These are available on all platforms, except Android.
Alternatively, you can find config.txt somewhere on your system's files:
- macOS / Linux / Raspberry Pi:
You can see a list of the available configuration files here.