Global GUI is the default one. only one GUI map file will be maintained for all the tests. you have to save the GUI file and load that into memory when ever it is needed by the application. memory will be saved. even when the application got any kind of changes, you need to modify only one file, which is easy.
GUI map file per test; you have to set this in General options. ever test will have a separate GUI map file independently. the files will automatically be saved and loaded when ever needed by the application. no explicit saving is needed. you need to modify every file when the application got any kind of changes.