Mail folder file structure confusion

I'm only partially familiar with the ~/Library/Mail/ file structure that supports displayed mailboxes in Mail.app. This question is about the locally stored mailboxes in ~/Library/Mail/Mailboxes. (I have one each IMAP & POP accounts, with their corresponding IMAP-username@imap.isp_name.com & POP-username@mail. isp_name.com folders, but this question does not concern them per se.)

As I understand it:

1. White mailboxes in Mail's list contain only blue mailbox subfolders & correspond to ~/Library/Mail/Mailboxes/ folders with no suffix.
2. Blue mailboxes in Mail's list may contain both subfolders of their own & individual messages, in which case in the file structure there may be a "parent" folder with no suffix & a folder.mbox child, the latter containing the individual messages not in that folder's children.

Is this correct?

If so, why do I have the following two folders at the same level in ~/Library/Mail/Mailboxes/:
Pxxxx.mbox <-- contains a Messages folder & Info.plist file
Pxxxx <-- empty folder (0 KB)

I have a blue Pxxxx mailbox in Mail, a root level 'on my Mac' folder containing the messages that correspond to the first folder above, but that's it -- it has no children folders.

I've had this mailbox for ages & the Pxxxx.mbox folder has dates consistent with that; the Pxxxx folder was created yesterday, but I don't know why.

Is is needed, useful, a sign of an error, or what? Can I delete it without ill effects? Why is it there?

Thanks for any enlightenment you can shed on this.