class 125
(n.) A group of UL classifications for containers that maintain inside temperatures less than 125 degrees Fahrenheit when exposed to a fire of specific temperature and duration. The times and fire temperatures are:
½ hour 1700 degrees F.
1 hour 1700 degrees F.
2 hour 1850 degrees F.
3 hour 1925 degrees F.
4 hour 2000 degrees F.

class 150
(n.) A group of UL classifications for containers that maintain inside temperatures less than 150 degrees Fahrenheit when exposed to a fire of specific temperature and duration. The times and fire temperatures are:
½ hour 1700 degrees F.
1 hour 1700 degrees F.
2 hour 1850 degrees F.
3 hour 1925 degrees F.
4 hour 2000 degrees F.

class 350
(n.) A group of UL classifications for containers that maintain inside temperatures less than 350 degrees Fahrenheit when exposed to a fire of specific temperature and duration. The times and fire temperatures are:
½ hour 1700 degrees F.
1 hour 1700 degrees F.
2 hour 1850 degrees F.
3 hour 1925 degrees F.
4 hour 2000 degrees F.

Source: Lock Industry Standards and Training (LIST) Council