I've been noticing that a lot of today's Poser content makers don't seem to know what these file types are and both in freebie and store products are mixing them up. This little trivia does make it difficult when a newbie tries to learn how to handle the product they just purchased or just downloaded and does make it more difficult for the existing Poser user to manage the ever growing library system.
So I thought I'd spell it all out...of course you can call whatever you want whatever you want but I would think that existing makers would like to show their superior knowledge and new and uprising creators would like to learn how to do it right and make their product easy for their customers to handle and enjoy.

So:
MOR is a Morph file, it can be a PZ2 or a CM2 but in either case is 'calls' upon the geometries, or morph, that is already existing within the CR2 code. It's the system created to easily apply a morph to say Possette or Dork, or any other character that has that morph already embedded within it's CR2 with a simple click.
It does NOT "inject" anything or apply textures.

INJ and REM is the newer system of Injecting or Removing morph data and 'calls' upon information from a designated path to it's corresponding ChanVis and Deltas folder (usually located somewhere within the Libraries folder but could be where ever it's created so chose to put them). So an INJ and REM file is the newer MOR method but works Completely different.
They do NOT inject and remove textures, that's what MAT files do.

MAT files apply textures to the figure with a simple click. They can also be PZ2 or CM2 files but in either case they apply the Materials to the CR2. A MAT file can also be used to remove all textures and apply all white to the material regions as well. Now there is also a hybrid called a MAT-MOR that can apply the materials And also apply the embedded morph simultaneously too!
A MAT or MOR file does Not have anything to do with the ChanVis and Delta files.

NULL now this file is neither a PZ2 nor a CM2 but is actually a CR2. A NULL figure is a special figure designed to prevent cross-talk between like characters within a scene, like say 2 Vickies or 2 Mikes.
NULL files have nothing to do with applying textures or morphs or Deltas or ChanVis.

So in short:
MOR applies existing CR2 morph data.
INJ and REM injects or removes morph data that is called from a special 'holding folder'.
MAT applies or removes textures.
NULL is a special intervention character to stop cross-talk.

Take it or leave it for what it says, but I have recently been seeing INJ miss-used as MAT and visa-versa and NULL used as a MAT that sets a character to all white and MOR miss-used in place of INJ/REM and so on and so on. Either these makers just don't know or they just think they are so Superior to the inventors of these systems that they could care less. But all in all it makes it very difficult for new users to learn and for those with HUGE libraries, a lot of unnecessary file editing to rename things correctly for future use.

applause

Oh, did I mention that PZ2 (or CM2) can also be written to simultaneously apply morph data, apply textures, And pose a character? Such as: MyCharacterFULL.pz2

I've added to this list, mostly for my personal use although I have seen others use them; SET is a pose file that does all those things not covered by MATs and MORs; making body parts or handles invisible, modifying joint parameters, etc. MAT-DIV (possibly, shortened to DIV) is a specialized MAT pose that instead of setting a material, sets an actor or actors to a custom material. I included MAT-DIV poses to make hosiery on the PT Girl's legs in my last clothing set. Also, when you've got a Runtime pushing a gig, it is nice if the function of these poses can be put into the name; aka Big_Boob_INJ.pz2