eclipse - The "Order and Export" tab in "Java Build Path"

Question

What's the use of "Order and Export" tab in "Java Build Path" dialog box? By the name 'order', I can guess that JRE System library is searched before Plug-in Dependencies, but why do we have 'Export' in Build Path ? And what's it for the button left side of the elements?

ADDED

It's the example that I borrowed from Eclipse distilled.

The orderes.webapp depends on orders, but it's likely to use the classes defined in ubl and catalog.

In such a case, orders can export catalog and ubl so that the packages that depends on it can use catalog and ubl.

And something more about "Order and Export".

1. The check of the left side button modifies the .classpath file to add exported=true such as <classpathentry exported="true" kind="lib" path="lib/log4j-1.2.7.jar"/>.
2. The src is always checked, and you can't uncheck it.

References

• Interpreting eclipse .classpath file. What does 'kind="con"' and 'exported="true"' mean?
• Configuring an Eclipse Java Project
• Java Build Path

Answer 1

This tab seems to pull double duty (see bottom of this help page for small amount of detail).

On one hand, it functions as the order of resolution for resources used in the building of the project in question (the "Order" part). On the other, it also serves to define what parts of this build path are contributed to dependent projects' (those listing this project in the "projects" tab for their view of this preference panel) build paths (the "Export" part).

This latter function is why the checkboxes exist on the left hand side, as you ask (note that the '-' signs cannot be unchecked, as source files defined in this project cannot be deselected, as stated in help file). A dependent project that expects some resource from the contributing project will not compile/run if the contributing project doesn't export that resource.

EDIT: see this SO post for corroboration.