Back before 3dsmax, 3D Studio had a plug-in mechanism known as IPAS. The acronym stood for IXP PXP AXP SXP, the 4 types of plug-in available for 3DS. They covered the four main areas of extensability, Image Processing, Procedural Model generation, Animated Stand-in control and Surface Procedural. Later, when 3DS 3.0 was released, new plug-in systems were added, BXP and KXP - Bitmap and Kinematic extensions.
I cut my C programming teeth trying to write my own little set of plug-ins for 3DS.. although I never got round to officially releasing them. It was only many years later a guy named Robert Seidel contacted me after playing with my 3dsmax plug-ins asking if I had ever written some free IPAS routines, as he was compiling a little archive of all the 3DS freeware. Until now, his site was the only place to download them!
I can't imagine anyone being able to run them, or need them, but they're stuck here for my own edification... and so I don't lose them in a pile of back-up CDs.