The build number that BuildMaster should use for the build. Defaults to the BUILDMASTER_PACKAGE_NUMBER variable populated by the "Select BuildMaster Application" action. Leave the field blank to have BuildMaster use it's own PackageNumber - the BUILDMASTER_PACKAGE_NUMBER will be set to the actual BuildMaster build number used in this instance.
If supplying a build number to BuildMaster and the build will fail with a BadRequest exception if an attempt is made to reuse a build number from a previous build. If this happens you will need to update the Jenkins build number to something greater than the latest BuildMaster build - there is a plugin to help with that: Next Build Number Plugin.