If using an svn/p4 source control method, they probably branch mainline into a release branch, test, then branch to a staging, test against production data, etc

If using an svn/p4 source control method, they probably branch mainline into a release branch, test, then branch to a staging, test against production data, etc, all the while fixing any necessary bugs. This allows development work to continue on mainline while keeping the release testing continuous and stable.