In patch 1.14 ClassicEra was rebased upon TBC, and they use the same packet structures, but the exact build numbers on which structure changes from retail are merged into each branch differ, as Season of Mastery and Burning Crusade phases are not released on the same day. Sometimes they are months apart, and there are no changes to structures made while a phase is live, only at the release of the next phase. So if a given phase of classic lasts a long time, it can end up being behind the other branches in terms of packet structures. This necessitates a way to specify the client branch for version checks in WPP. Also the build numbers given to clients seem to be global (a build in one branch increases the build number of subsequent builds of other branches).
In patch 1.14 ClassicEra was rebased upon TBC, and they use the same packet structures, but the exact build numbers on which structure changes from retail are merged into each branch differ, as Season of Mastery and Burning Crusade phases are not released on the same day. Sometimes they are months apart, and there are no changes to structures made while a phase is live, only at the release of the next phase. So if a given phase of classic lasts a long time, it can end up being behind the other branches in terms of packet structures. This necessitates a way to specify the client branch for version checks in WPP. Also the build numbers given to clients seem to be global (a build in one branch increases the build number of subsequent builds of other branches).