In these settings, you are able to change the Network Selection to be Automatic or Manual. When this is changed to Manual, you are given a choice of which network you want to choose. I have left this Automatic but if you want to avoid roaming, I guess you can lock it to, say, Vodafone if you are an Australian 3 customer, to avoid roaming to Telstra for data.
The Network Mode seems to be where I have had a little breakthrough. I changed the setting from "Dual" to "3G" as "GSM" is much slower than 3G from what I understand. The impact seems to have been that I am hardly ever getting no signal when I am trying to call someone. I am not sure whether this is because of this change in settings, but time will tell. It seems that the N9 seems to struggle with deciding at times between whether to make the call 3G or GSM and when you are ringing someone, the phone sits in silence for about 10 seconds and says "could not connect - Redial??"
Let me know what your thoughts are and if this has helped you, leave a comment below. Would love to get some verification from another user.