A - Adds your signature. This is useful for commenting in the Talk pages, since you don't need to write your nickname and make a link to your profile every time you are making a comment. During the editing, you will see the signature as a number of hyphens and tildes. But don't worry, these will get replaced by your signature and a link to your profile after saving the page.
Note: This applies only to Talk pages. Toolbars for editing normal articles will not feature the Add signature button at all.
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