Really can mean almost anything. As well as what is posted above; some B stock is actually A stock that they have lower the warranty period, so they can then lower the price, in order to clear out overstocks. Not necessarily 30 days. I've seen many B stock items where the A stock's 3 year warranty is lowered to 1 year. Still a lot of time.
Just keep in mind that if it is B stock that was refurbished; it was A stock before it was sent back. In my experience, the few things I have owned that broke, were just as likely A stock than B. It seem sometimes that when A stock breaks, we don't react the same as when B stock breaks.