I see a lot field definitions around that unnecessarily include a SourceID attribute:
More often than not, the value of this attribute is set as above. Microsoft's own documentation samples tend to include this attribute.
So what does it do? Not a lot as far as I can tell; I don't include it with my own field definitions because inspecting the field after deployment with SharePoint Manager proves SharePoint sets it automatically with the GUID of the list that created the field. The Field Element documentation also indicates it is optional.
If you're intent on using it, the documentation describes this attribute as "containing the namespace that defines the field, such as http://schemas.microsoft.com/sharepoint/v3" so rather than use the default namespace it may be advisable to use your own (I haven't tried this myself).