Any API Tools enabled project in your workspace can be compared to any one of the API baselines set up in your workspace. In addition to selecting projects to compare to a baseline you can also select (or multi-select) any of the following within an API tools enabled project:
NOTE: all selections must be made from the Package Explorer because the compare wizard works only for Java elements.
To compare to an API baseline do the following:
Setting up a baseline
API Baselines Preferences
API Baseline Wizard
Missing API Baseline Quick Fix