In some jobs you accrue paid days off after working a full week on 24/7 call duty. That duty usually involves full phone availability and even going on site if the incidence cannot be solved remotelly. The paid off days you accrue you can then use at your convenience and are not deduced from the PTO. Usually the reason for that policy is because the 24/7 availability duty could potentially be physical or mentally extenuating and can be simply used to rest.
A literal translation from my native language would be "compensatory days", but I would like to know the commonly used english phrasing if such exist.