This is an exerpt from the Patching primer found on the newbie page. It briefly explains what each folder is used for.
Patch Folders info from patching primer
This is the default directory for patches. Any file put in here will be used ahead of all other files, in both single and multiplayer games. Files in Simpatch are used ahead of the same named file in Shellpat for SP; however, files in Multi will over-ride Simpatch for MMP.
This directory is only used by the single player game, so it is a good choice for e.g. flight and damage models, which will get you ejected from a server in MMP if you have changed them from stock. It is also a good choice for any date based patches, otherwise you will get some weird results in MMP. I had a lot of allies flying Siemens-Schuckerts at one point.
This directory is only used by the game for MMP. It over-rides Simpatch.
This directory operates in much the same way as Simpatch, and is mainly used for 3D shapes and terrains. I don't really know why it is there; I use it solely for terrains. Though it works in both SP and MMP, I'm not sure if it over-rides the Shellpat and Multi folders or not.
Read the read me info in any patch this is just a brief explanation of the patching folders and what they do.
Campaign Manager required for some patches like WFP read the read me or FAQ on patches site
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