To compare two revisions select the two revisions in the Show History Dialog, right-click and choose the 'Compare Revisions' menu. This will launch the Differences Tool as described in the section called “Layout”.
To view the changes for one revision, right-click the revision and select the 'Compare with Previous Revision' menu. This is the equivalent to selecting this revision and the previous revision and choosing the 'Compare Revisions' menu.
To compare a revision with your local file, right-click a revision and choose 'Compare with Local File'. So if you have checked the file out and made changes to the file, these changes will appear here.