The X server maintains three different selection buffers, PRIMARY, SECONDARY and CLIPBOARD. The PRIMARY buffer is usually used for copying and pasting via the middle mouse button. And the CLIPBOARD buffer is used when the user selects some data and explicitly requests it to be “copied” to the clipboard, such as by invoking “Copy”.
The VirtualBox client tool synchronizes the Windows clipboard content to the PRIMARY and CLIPBOARD buffer of a Linux host. But it only synchronizes the CLIPBOARD buffer back out to the Windows host. The reason why it’s not using the PRIMARY selection is that by only selecting a text in Linux overwrites the Windows clipboard immediately, which is an unexpected behavior for Windows users. At least that’s their excuse.
It might not be the expected behavior for Windows users. But it’s definitely not the expected behavior for Linux users. The forums are filled with posts like “Copy/Paste from Linux guest to Windows doesn’t work”. It is especially annoying that the selected text from a terminal window can not be easily pasted out to the Windows host. There are a couple solutions. But none are really satisfying.