The difference is the third measuring channel for measurements of a tertiary winding.
The WR14, WR50-12, WR50-12R, and WR100-12R models are capable of performing two-channel measurements, allowing for measurements of both the high voltage (HV) and low voltage (LV) coils on a single core leg.
The WR50-13, WR50-13R, and WR100-13R models are designed to conduct three-channel measurements, allowing for measurements on the high voltage (HV), low voltage (LV), and tertiary coils on the same core leg.