In AutoCAD 2006 and up, Tab means AutoComplete.
Do you have trouble remembering those arcane AutoCAD commands or system variables? Do you hate typing in those long command names?
Type the first few letters of the command or system variable desired and press the TAB key. AutoCAD cycles through all the commands that start with those letters with each press of the TAB key. When you get to the one you want, press Enter.
The more letters you enter initially, the fewer matches AutoCAD will need to cycle through.
Bonus: If you overshoot the desired command, use Shift+TAB to back up.