With an XP machine on a Windows 2000 or 2003 domain, you can just leave them at the defaults, ie as they are shown in your screen shot. Once you've joined the domain, the DNS Suffix will automatically default to the domain you've joined, in your case test.net and the NETBIOS name will be the same as your computer name pro10
NETBIOS is really only used with legacy (Pre NT) machines such as Windows 98.
You get 2 things from DNS... name resolutions & service announcements. DNS tells you that www.msn.com equals some IP address. It also tells you which server to go to to logon to your domain and where to find a Global Catalog server when you're browsing AD objects.
Changing your local DNS suffix affects none of these things. All changing your local DNS suffix does is to automatically append that suffix to host names when talking on the network.
For instance... if my DNS suffix is ACME.COM and I type in "ping server1" I will get a reply that says "Pinging server1.ACME.COM". My machine then contacts my DNS server and asks it how to get to server1.ACME.com to which the DNS server replies with the IP address of server1 in domain ACME.COM.
So... since you access resources most of the time in a single domain it makes sense for you to make that your primary DNS suffix. It doesn't mean that you cannot resolve names in other DNS domains, it just means that the first one it tries... the one displayed to you... is the primary.
Does that help any more? Changing your DNS suffix doesn't have anything to do directly with Active Directory. If you mess up your name resolution you might not be able to connect to a particular resource though.
The default configuration is to use your local AD domain name in this fashion as well as to check the box. This is because you are expected to be accessing resources in your own domain most often. In addition, it is expected that other devices trying to connect to your computer would be expected to be in your own domain.
The only reason why you would want to use a domain other than your own default would be if you expected devices in another domain to connect to your computer and you wanted to make sure your DHCP-enabled computer registered a name within the OTHER DNS domain name in addition to or instead of your own AD DNS domain name.
So, unless you're publishing data on desktop computers to people in the other domains don't worry about this. If someone in another domain needs to be able to connect to a server in your domain you may need to put a static entry in DNS for them to be able to connect by name, but you probably won't even need to do that.
Workstation1 in DomainB registers it's primary suffix as "DomainB" but registers an additional DNS suffix for "DomainA". Workstation2 queries DNS for "Workstation1.DomainA.com"