On the Includes tab of the Paths and Symbols section of the Project Properties dialog in the Eclipse CDT, there is an "Export" button:
The best documentation I have found says that this "toggles whether the selected include path is exported or not." If I click it, it changes to "Unexport" and "[exp]" is appended to the selected include path.
What exactly does this do? What does it mean to "export an include path?"
This button makes the path in question available to projects which reference your current project. Project references can be configured using the References tab in the same "Paths and Symbols" group shown in your screenshot.
This functionality has been improved in the next version of CDT (7.x). It allows you to create dependencies between projects and have include, library and library files being propagated automatically. (In the next version you can alter the order of the referenced projects...)
See also the Export Settings page which can be enabled with
Window > Preferences > C/C++ > Property Page Settings > Display "Export Settings" Tab