Baselines
A baseline captures the state of selected items and their relationships within a project at a certain point in time.
Baselines are automatically created when someone initiates a review. Anyone with read/write permissions can manually create baselines.
When a baseline is created, the current version of each selected item (and relationships, if selected) will be associated with that baseline. By creating a baseline at each project milestone, you will be able to view the status of your project at that particular point in time.
Note
In order to create baselines, versioning must be enabled on the project.