At least in my experience, there was a shift around the late 90's at US corporations away from having a set number of no-questions-asked paid sick days (which often went unused) plus a fixed amount of vacation (which you were strongly encouraged to use); to having a fixed number of "Personal Time Off" days (PTO) accruing every month, and anything you did other than work used them up.

This, predictably enough, had the effect of people coming to work sick, because if you took the day off you'd lose a vacation day.

Now it seems the new trend is to have no specific number of free days, but leave it up to the employee to take however many days off he/she feels are necessary. Then it's up to their manager to deal with it if they take "too many."

This lets you tell people they can take vacations, while not carrying any liability to pay them for the unused vacation if they quit or are laid off / fired.

The downside is that there is no longer any incentive to take any specific amount of vacation (after all, it's not accrued and can't expire). So if you're really busy, maybe you'll never take more than a couple days at a time.

The upside would seem to be that if you're a top performer and manage your time very well and have a cool manager, you can potentially take a LOT of time off without violating any company policy. I have however not heard any stories of this being done.