In TFS 2010, Build Number Format in Build Definition uses date format by default. For example, here is the Build Number Format.
The build number is incremental on Dec 31, 2012
If our project would like to use Major release 2 and Minor Release 1, the following build number format can be used easily.
Can we put the following Build Number format? The answer is no because the following screenshot will appear.
You can modify the Build Process Template (xaml) to meet your need of Build Number format as well.