I work with motor controllers, so in my world, UL 508 is for Industrial Control Equipmnet, specifically, direct on-line motor starters and reversing motor starters. UL 508C is called Power Conversion Equipmnet, and in my world it realtes to variable speed drives, which are motor controllers that can be used to adjust the speed of a motor. They use PWM or switching power supplies, so the load (as seen from the utility presepctive) varies, and they are very noisy EMC wise.
Section 44? well, lets have a look...thumb....thumb.... Oh! this is one of the friendlier sections. It has to do with dielectric withstand testing. It describes how the equipment must withstand without breakdown for one minute between uninsulated live parts and the case, between isolated secondary circuits and uninsulated live parts and the case, between primary circuit uninsualted live parts and isolated secondary circuit unisulated live parts.
The test potential shall be AC or 1.4.14 times the the follwing values for DC:
for equipment rated not more than 250 volts and intended for use in pollution degree 2 its 1000 volts
for all other equipment 1000 volts plus twice the rated voltage.
Then it goes on to describe the test equipment, it must be 500 volt amperes or larger transformer, has to output a sine wave, has to be adjustable, and has to be able to rise at a uniform rate.
In the past I have used Assoicated Research hypots to do this test, and the UL witness had no issue with the gear. They pop up on E-bay from time to time, and just about anyone can do a calibration on one.
Naturally, I am paraphrasing here, but this is the general feel of the thing.