Where I've worked, this was addressed somewhat loosely. You have PTO. You commit to scheduling time off in advance to the greatest degree you can. The employer commits to accommodating those requests to the greatest degree they can.
If you need to miss work unexpectedly, you do so. The employer just trusts your judgement that it's necessary. If it's going to be more than a day or two, you are expected to talk with your employer and explain what's up -- mostly so the employer can adjust assignments and schedules with a better understanding of what to expect.
Do some employees abuse that? Sure. But, in my experience, that's rare and the people doing it are usually poor workers anyway. They can be handled through whatever the normal review mechanisms are.
This is my most common experience. I don't know how common it actually is, and I'm sure other situations would require different, perhaps more formal, procedures. But it's worked well in the places I've worked.
