The items enqueued into a pipeline are each associated with a git ref. That ref may point to a proposed change, or it may be the tip of a branch or a tag. The triggering event determines the ref, and whether it represents a proposed or merged commit. Zuul prepares the ref for an item before running jobs. In the case of a proposed change, that means speculatively merging the change into its target branch. This means that any jobs that operate on the change will run with the git repo in the state it will be in after the change merges (which may be substantially different than the git repo state of the change itself since the repo may have merged other changes since the change was originally authored). Items in a pipeline may depend on other items, and if they do, all of their dependent changes will be included in the git repo state that Zuul prepares. For more detail on this process, see Project Gating and Cross-Project Dependencies.