For the preparation of the beta release (see BetaProcess), all uploads are held in the queue and are subject to manual approval of the release team. Packages that affect any images (flavours included) that will be shipping for this milestone require release team approval before uploading. One can use the seeded-in-ubuntu tool from ubuntu-dev-tools to help figure out if a package will affect an image. Packages that do not affect images will generally be waved through the queue as time permits.
The purpose is to stabilize the archive, allow the archive administrators to consolidate it and fix package inconsistencies, and be able to fix critical bugs quickly and in an isolated manner.
All exceptions require the approval of the release manager (see FreezeExceptionProcess), which is generally only granted if the upload fixes a bug which is milestoned for the beta release and does not have any unrelated intrusive changes.