You use that dialogue box the specify the tab attributes - position, left/right/centre etc., leaders. The empty box is where you type the number of in/cm or whatever units you're using for the tab-stop position.
See: https://support.office.com/en-us/art...#ID0EABAAA=Mac
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Cheers,
Paul Edstein
[Fmr MS MVP - Word]
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