header path search rules and Xcode settings for ObjC

sometimes i get external modules to integrate using the <path/file.h> form for header file paths (even tho they are not apple framework or system files). sometimes i get them with the "path/file.h" form. sometimes they work. sometimes they don't. there are many header file settings in the xcode build settings. i count at least 11 settings for header search paths in xcode. there may be more. does anyone have a definitive Apple guide to all this complexity with respect to ObjC headers in one place? every time there are header file search problems, it seems to involve a random process of changing various settings until everything builds, but that seems to be the wrong way. it also takes a long time.