Oddly my dictionary doesn't give either of the definitions I associate with these three letters. To me they stand for Take a Break. I'm not terribly keen on the weekly mag, but TAB's Fiction Feast is great. When writing stories I indent paragraphs using the tab key (then take the indents out again for TAB!)
See where it says 'Top Twist' on the cover of the current issue? That's my story they're talking about!
Non writers probably think of a tab as a drinks bill (oh, OK writers might be familiar with that one too) A a small strip of material used for hanging up clothes is a tab and tabbing is the action of attaching these. A stage curtain can be known as a tab and it might need to be tabbed in order to hang it.
To keep tabs on someone is to keep them under surveillance.