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The Build Version Expression.
This field allows you to use Query Syntax in order to get dynamic values in your version string. Note that the reuslt of this expression should be four 16 bit integers, by conventions this would be (for .net) Major.Minor.Build.Revision or (for native apps) Major.Minor.Release.Build.
Note that Continua sets the Build Version string when the build is started, using the Version Format String specified in the Configuration Wizard. You should only need to use this Action if you need to apply a Version string that is generated during the build (for example by reading a value from a file).