PM system

[chaoskreator Jason]

I'm wondering if anyone can explain a little of the phpbb private messages system to me. I see there are 4 tables for PMs; privmsgs_rules (self-explanatory), privmsgs_folder (which I'm assuming is user-created folders), privmsgs_to (which I'm assuming is just a table to contain where each message originated, where it's going and what happened to it), and the privmsgs table (which actually contains the messages, themselves).
What I'm confused about is in the privmsgs_to table. Some of the entries have -1 in the folder_id field. What does this denote, exactly?

Any tutorials or articles on Private Messages anyone could provide would be greatly appreciated.

[topdown]

I don't know of any tutorials explaining the actual logic for the PM's.

But without looking I assume the folder_id is an int, and it is probably the default folder Inbox.
I can look it over tomorrow if you are still interested in knowing.