The predefined elements are part of the Element Mapping feature. The idea is if you deal with an application still in development you can just predefine your elements to build the test cases earlier. Then once application development is completed you just connect each predefined element with the actual one in the application similar to recording experience and your tests are ready to execute. You can read more about that approach here
As to elements being global, they actually are since you can access each element from each test case, including from code, all within the scope of the project. The Elements Explorer organizes the elements per pages and frames though so it identifies the page/frame specifics to group the elements accordingly. You may want to look at the pages merge logic described here
to understand the merging specifics if you want to control that.