Runtime Rundown
A graphical version of where Poser stores files is available as well.
Runtime\Geometries - This contains all the meshes for Poser. Everything from the default figures that come with Poser to all the clothing, etc. that you create, purchase, or download as a freebie. You'll find all sorts of subfolders in here that contain the meshes. You normally don't want to fiddle with the folders and files in here unless you want to run something through UV Mapper to create a template. If you do take a mesh from here and modify it in another program, please remember that you can not redistribute that changed mesh without the original creator's permission.
Runtime\libraries - here's where you'll spend most of your time. Below are the important subfolders broken out. If you have P4 or PP, you can only create one level under each folder. If you have P5, you can go as deep as you want to. (All file extensions are shown for P4/PP/P5. If you have P5 and have your files compressed, then substitute z for 2 at the end of the extensions - .pz2 = .pzz, etc.)
Runtime\libraries\!DAZ - if you bought any of the version 3 Mil figures and their morph packs, you'll see this folder. This contains all the morphs that you inject into them. If you have Poser 5, make sure to keep this folder in the default Runtime folder!
Runtime\libraries\Camera - This contains all the camera presets. Some people who create sets will save camera settings so it's easier to use the set. The files are .cm2 files. (Note: some people have saved MAT files as .cm2 files because the Pose folder is overflowing with pose and MAT files. I hate that, but I can see why it's done. You can always rename the file to .pz2 and move it - and the thumbnail - to somewhere in the Runtime\libraries\Pose folder.)
Runtime\libraries\Character - This folder contains all the figures, such as Vic, Mike, etc. It also contains most clothing, some hair, animals, etc. It depends on how a mesh is set up as to whether or not it is a Character, Prop, or another type of file. These files end in .cr2.
Runtime\libraries\Face - This contains all the facial expression poses. .fc2 is the file extension.
Runtime\libraries\Hair - This contains all the Hair meshes that were saved as hair props. (Hair props contain special information and will be deleted when you delete the base figure.) Hair files are .hr2.
Runtime\libraries\Hand - This contains all the hand poses - .hd2. Runtime\libraries\Light - This has all the light settings. Lighting is very important for good renders and if you create a useful light set for a scene, you might want to save it. DAZ has some very good light sets in the Plat Club, and RuntimeDNA has some very nice free ones. .lt2 are light files.
Runtime\libraries\Materials - P5 only. Any materials that you create in the Material Room are saved in here - .mt5.
Runtime\libraries\Pose - Poses that affect the full body are saved here. MAT files (which tell Poser what materials to apply to the figure) and MOR files (which tell Poser what morphs and what values to use) as well as INJ and REM files (Mil figures, v3 use INJ/REM files to add/remove the morphs in the figures) are all contained here. It tends to get very crowded very quickly. .pz2 is the file extension here.
Runtime\libraries\Props - This contains all the props, usually jewelry, weapons, furniture, etc. These files are .pp2.
Runtime\prefs - This contains the Poser set up information, things like screen size, tool bar locations, etc. If you have Poser 5 and can not see the thumbnails in the library, you can close Poser and edit the previewNum value in Runtime/prefs/LibraryPrefs.xml - change it to something higher than 100.
Runtime\python - This is where any python scripts go, somewhere in the \poserScripts subfolder. (For Poser Pro and Poser 5 only. Poser 4/Artist does not handle python scripts.)
Runtime\ReadMe's - This is where most of the readme files are located for different items, things like how to use an item, terms of usage, etc. Sometimes you'll also see Runtime\ReadMe, Runtime\Read Me, etc.
Runtime\textures - This is where all the texture maps, transparency maps, etc. go. Unless you want to edit MAT files, don't change things around in here. If you want to create your own texture maps, you can create a new subfolder in here and put your maps in there. (You can create subfolders in here, but careful not to get too many levels deep or Poser might have problems finding the files.