They want to know if it is a 3 or 4 series carrier.
if you have 3.07- 3.73 gears you need a 3 series carrier. If you have 3.73 or higher gears you need a 4 series carrier.
The difference is in the offset of the carrier putting the ring gear farther from the pinion center so the pinion can be a larger diameter and have more teeth allowing the higher gear ratios.
Sometimes ther are aftermarket gears that can put the 4 series gears on the 3 series carrier. 3 series carriers are more common and cheaper.
To determine the gear ratio there are numbers on the ring gear like 41-10 the 41 is the ring teeth count the 10 is the pinion teeth count. This would be a 4.10. divide the larger # by the smaller # to get the ratio.