The Symbols Settings is used if you need to apply some particular settings on specific symbols (risk factor, max order size, copier status) or you need to manage a specific mapping of symbols between your masters and slaves accounts.
For example, your strategy requires XAUUSD to have a risk factor of Fixed Lot .05, but the rest of the other symbols you are trading on will follow Auto Risk 1. You can make that exception in the Symbol Settings. Here, you would enter XAUUSD and simply enter the risk factor of Fixed Lot 0.05.
Important Reminder: You have to enter the exact symbol (case sensitive) of the master and the slave in the mapping, don't forget to select your master in the drop-down list in the symbol settings.
Symbol Mapping is meant to be used for mapping complex symbol differences between Master and Slave or if you wish to reference a specific symbol to a different one, while Symbol Setting is used if you want a specific symbol to have a different setting.
Both functions are synced together, so the changes you made on Symbols mapping will also reflect on Symbol Settings and vice versa.