The goal of UEx is to deliver UltraEdit for Linux while ensuring we do not lose the native Linux experience. Primary Selection support - that is copying and pasting via the middle mouse button - is one feature that Linux users must have, so UEx supports it. Primary select is quite easy, simply select text in the edit window, navigate to an internal/external text field and hit the middle mouse... Your text is there!
Primary Select is a native Linux function which copies the active/highlighted text to the clipboard and pastes it when you click the middle mouse button. So to use the feature, select the text that you wish to paste.
Now, navigate to the text field (either internal or external to UltraEdit) and click the middle mouse. You will see that your text has been pasted into the text field.
As we've already mentioned, the feature works in external applications as well.