It's not from the documentation...this is a question from which I see in the actual MySQL table structure.
Here is a snipped of the w3t_Posts table to show you:
<pre>
[PRE]
mysql> describe w3t_Posts ;
+--------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+------------------+------+-----+---------+----------------+
| B_Board | varchar(100) | | MUL | | |
| B_Number | int(11) unsigned | | PRI | NULL | auto_increment |
| B_Parent | int(11) unsigned | | MUL | 0 | |
| B_Main | int(11) unsigned | | MUL | 0 | |
| B_Posted | int(11) unsigned | | MUL | 0 | |
| B_Last_Post | int(11) unsigned | | | 0 | |
| B_Username | varchar(30) | | MUL | | |
| B_IP | varchar(60) | YES | | NULL | |
| B_Subject | text | YES | | NULL | |
| B_Body | text | YES | | NULL | |
| B_Mail | int(1) unsigned | YES | | NULL | |
| B_File | varchar(100) | YES | | NULL | |
| B_Kept | char(1) | | | | |
| B_Status | char(1) | | MUL | | |
| B_Approved | char(3) | | MUL | yes | |
| B_Picture | varchar(100) | YES | | NULL | |
| B_Icon | varchar(30) | YES | | NULL | |
| B_Color | varchar(10) | YES | | NULL | |
| B_Reged | char(1) | YES | | y | |
| B_UTitle | varchar(50) | YES | | NULL | |
| B_Counter | int(9) unsigned | YES | | 0 | |
| B_Sticky | int(11) unsigned | YES | | NULL | |
| B_Replies | int(4) unsigned | YES | | 0 | |
| B_Poll | varchar(200) | YES | | NULL | |
| B_ParentUser | varchar(20) | YES | | NULL | |
| B_UStatus | char(1) | YES | | NULL | |
| B_Topic | int(1) unsigned | | | 0 | |
| B_Convert | varchar(10) | YES | | markup | |
| B_Signature | text | YES | | NULL | |
| B_LastEdit | int(9) unsigned | YES | | NULL | |
| B_LastEditBy | varchar(64) | YES | | NULL | |
| B_PosterId | int(9) unsigned | | MUL | 0 | |
| B_Rating | varchar(5) | YES | | 0 | |
| B_Rates | int(4) | YES | | 0 | |
| B_RealRating | int(1) | YES | | NULL | |
+--------------+------------------+------+-----+---------+----------------+
35 rows in set (0.00 sec)
[/PRE]
</pre>
I used this table as the example because it has the most instances of the "MUL" key in it, although others do too.
Cheers, Bill
