Right, up through the end of Pegasus Mail V3.x at least, and possibly into the earliest V4.x versions (I'm not quite sure), whenever the user invoked a "selective mail download" -- which I still do, all the time, first checking the message count using the ancient MultiPOP extension -- the *.TOP file, with a hex file name and containing the mail headers, would be written, temporarily, to the user's mail directory (okay, "folder", I'm old school . . . ) on the client machine. If the user had a lot of mail "up there", on the host mail server, the TOP file size could be substantial, 100Kb, even, at least under versions of Windows prior to XP.
That file should have been and was deleted upon completion of the mail download, unless there was something else going on, such as a client-side lockup or crash, or, sometimes, a winsock timeout.
Under the most-recent 4.x versions, I don't see that file in the mail directory anymore, so I presume that it's being written somewhere else, probably to a *.tmp directory somewhere. Under Win 3.1, when hard drives were small, one needed to monitor such orphaned files. But as other posters suggested, it really doesn't matter much anymore, except to the observantly curious among us.