There are three different search paths in XCode Build Settings:
Could
These are used for searching for Frameworks, Header files, or Libraries that are not in the system search paths (like QTKit.Framework, standard C++ header files, etc).
My most common use for this is using the boost header library (*.hpp) files in my code I add the relative path "../lib/Boost/1.46.1" to the Header Search Path.
I find it better to add this at the project level instead of in each target. That way the targets inherit this and it only needs to be changed in one place if I update the version of boost.