Extension Housing Part Question??
 

http://www.subaruparts.com/diag/?mod...category=170-1

I'm looking on SubaruParts.com and I have noticed that you can buy a complete starter or you can buy it in many parts for seemingly more money than buying the complete unit.

Now I may be having my AWD worked on and I was just doing some checking on parts prices. I noticed that you can buy a "Complete Extension Case" and was wondering what this included (clutches, solenoid, gaskets, etc...) or if this is just another name for the case by itself????

Thanks