There are a lot more than 5 types, but the various Kaddes are for different applications. The nearly universal coupler from Kaddee is the #5. The #4 has a different shank on it, and I've found it to work well on steam engine pilots when there isn't room for the pivot loop on the #5 to fit. The #48 are Kaddees "scale" sized couplers. You will also notice that Kaddee has couplers in various # series (20's, 30's, 40's, etc.). What those are used for is specific applications. Sometimes a locomotive will have a longer pilot that requires a long shank on the coupler, so some models in each series will have a longer shank. On my f-units which run on a modular layout with a minimum radius or 36 inches, I can run reliably with the ABBA set close coupled to prototype distance. To get closer coupling, Kaddee offers short shank versions of their couplers. If you are running on 18-22 inch radius, you can't have the units coupled so close together, so you need a standard shank. Kaddee also offers underset couplers where the shank is mounted to the bottom of the coupler knuckle for locomotives or cars where the coupler height is too low. They also offer overset couplers where the shank is mounted to the top of the knuckle for applications where the coulpler mounting is too high. Finally, they offer the 20 and 30 series couplers with a hairpin spring and coupler pocket that allows the coupler to mount to locomotives where there isn't enough room behind the trucks to mount the longer #5 coupler pocket. I have never succeeded in getting the hairpin springs into the coupler box myself. I have used the 20 series or 30 series in some f units that didn't have enough room for the #5 type coupler pocket, but I didn't bother with the centering spring. I realise that this reply may seem long and rambling, but Kaddee has tried to make a coupler to fit every situation for every model produced. Oh I almost forgot, they also make some couplers with extra long shanks to fit Rivarossi passenger cars. I think if you go to their web site, they probably have a chart listing the couplers that they reccomend for each common plastic model locomotive or car. They also sell packages of shims in 2 different thicknesses. The thinnest ones are grey and the thicker ones are pink. You can use them between the truck and the car body to raise the car slightly to raise the coupler height.