What is the difference/purpose for these types of couplers?  Can they be interchanged? 

Thanks for any insight.

no, they are not interchangeable, but they will work together.

these are meant for situations where a regular coupler would either be too high or too low. knuckle type couplers are pretty unforgiving as far as having them all the same height goes. mismatched heights causes problems with uncoupling. the overset shank is used where the coupler box is slightly higher than it shouls be, the offset in the shank helps correct this. the underset shank does the same when the coupler box is too low.

an alternative fix for the tooo low coupler box is to use thin washers between the trucks and body to raise the car slightly, but this is hard to do on locomotives.

Good Morning all,

        What Jeff said is right on,  but let me say it a different way.   First,  remember,  the terms used here are under and over shank.   It is important that you understand we are speaking about the location of the coupler shank in relationship to the knuckle.
         Therefore,  and under shank has the knuckle sitting up higher and an over shank has the knuckle "dangling" down a bit. 
         If you need to have a coupler higher to match up with your coupler height gauge,  then you use the under shank or,  as Jeff said,  use washers between the car underframe and the top of the truck bolsters.
If you need a lower coupler,  then the over shank is the coupler of choice.
       BTW....notice I mentioned a coupler height gauge.   This is a MUST tool to own if you are using knuckle couplers.   Kadee makes three different kinds.   Two are metal,  one for standard gauge couplers,  and one for narrow gauge.   The third,  the 206 is a plastic version of the metal standard gauge height gauge and is the one that I recommend so that if you accidently leave it on the track,  your system,  either DC or DCC will not short out.

Good Morning : I find that it is a lot safer and easier to check coupler height on a seperate unpowered   section of track .  I usually have about three  feet of track on my desk to check the height & other relations of car to track & each other . J2

Gentlemen,

Thank you very much for the great explanations.