Top-fermenting versus bottom-fermenting isn't really a special quality of the yeast I don't think, it's dependent on the fermentation. Ales ferment warm and have a strong krausen (although in actuality it's the yeast floating around in solution that do the fermenting, not the yeast floating on top in the krausen). Lagers ferment cooler and so the yeast spends more time settled to the bottom 'cleaning up' and conditioning the beer. The terms indicate the type of fermentation the yeast is suitable for, but I don't think it really tells you anything more than simply calling it an ale yeast or a lager yeast. In other words - it's not something you have to worry about. As long as you aren't buying a yeast meant specifically for lagers, you're going to end up with a top-fermenting yeast suitable for ales.