Whenever you select *All as the source element, the Extended Analysis checkbox becomes active. If you select this checkbox then Extended Analysis will be performed of the databases on the target server.
Extended Analysis means that every design element in the target database will be analyzed to determine if it is an Orphan Element or an Extra Element (see Status Values for more information). Extended analysis will take longer than normal analysis, sometimes significantly longer, and is not always appropriate for every situation. However Orphan Elements and Extra Elements can be serious issues, so this feature should be enabled when applicable.