Wireshark-users: [Wireshark-users] What is "Export PDUs to File..." intended to do?

From: Guy Harris <guy@xxxxxxxxxxxx>
Date: Tue, 15 Jul 2014 18:05:19 -0700
Currently, it writes something to a temporary file, and then closes the current file and reads the new file in.

1) What do the four choices it offers mean?  I tried it with "OSI Layer 3" on an HTTP capture and no packets were written.

2) Why does it replace the current capture, rather than writing out to a new file with a specified name?  That's not what I'd expect a menu item that begins with "Export" to do.