Code::Blocks variable types synthesis
You will find here the different variable types available in Code::Blocks and when/how to use them.
Environment Variables plugin
These are system wide and can be set/overridden by the envvars plugin. It's helpful if your e.g. have another build systems besides C::B that uses envvars (like Makefiles). This way you can "share" this technology.
Custom Variables under Global Compiler
These are helpful to e.g. quickly change a path for a compiler suite. For example: You have gcc 4.2.0 and gcc 3.4.5 installed. They all have the same path structure so if you setup the main path to the executables as e.g "D:\Devel\GCC$(GCC_VER)" and additional include/lib "D:\Devel\GCC$(GCC_VER)\include"/"D:\Devel\GCC$(GCC_VER)\lib" folders as you can easily switch between both compilers by just changing the custom variable. This would (of course) apply to *all* projects that use GCC. Here is the detailed Global compiler variables page
Custom Variables under Project Build Options
These are very helpful if you want to compile your project against two compiler as stated above. You can have two targets with different compilers. In addition you can (as you have stated already) easily append a "d" to libs for the debug version, w.g. wxmswud. The values are being overridden in the order of details - compiler custom variables are overridden by project CV's and project CV's are overridden by target CV's. It only makes sense that way...
Where does Global variables fall in this order of precedence?
These variables have a very special meaning. In contrast to all others if you setup such a variables and share your project file with others that have *not* setup this GV C::B will ask the user to setup the variable. This is a very easy way to ensure the "other developer" knows what to setup easily. C::B will ask for all path's usually necessary.