In terms of TFS, a work item is a single unit of work, which needs to be completed. Review Assistant allows you to view related work items to files (revisions) that are added to a review.
The example workflow is described below.
Someone creates a work item that describes what should be done. In our case, some additional functionality should be added to the current project.
Thereafter the assignee writes code to realize required features.
The developer eventually makes the shelve operation to save changes. At this step, the developer adds related work item ID to the current shelve.
Before committing into a repository the developer creates a new code review to allow colleagues check the correctness of the code. The developer adds the files (shelve) to the current review. Review Assistant automatically shows the work item ID associated with the current shelve.
Double clicking the related work item will open the work item window. Thus, reviewers can quickly get on the inside.