You can define whether a customer can create tickets via the web interface or not. If they can you can furthermore define groups for which a customer can create tickets via web interface. "-" means all groups are available.
That means for example: if you are working with a dispatcher, you could implement a group "Income" and only select this group here. Thus, all incoming tickets are routed to this group and can be dispached to other groups from here.
In this example we selected "Sales" and "2nd Level" to be selectable for the customer.