That's the 'Combat' tab.
I know nothing about logging - can't imagine why I'd want to log an MMO - but I thought this might help. Best of luck getting an answer 
You should be able to untoggle the chat logging by using the slash command. Just type:
/logchat "fighting"
into the chat window and it should stop. (Substitute the real name of the tab for "fighting".)
...I forgot what experience means.
The chat logging system will also log ALL global channels you belong to, even if they're NOT assigned to any tab. >.<
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Also note that if you ZIP (or otherwise compress) your old logfiles they will be reduced in size quite a bit. I used to log all of my chat in EverQuest. I was a guild officer and raid leader and always needed to refer back to our chat. I had many gigabytes of log files, but each month I would put them in a folder and add it to a .ZIP archive. they were usually reduced 90% or more. Log files are perfect for this kind of compression because they are plain text with lots of repeated patterns.
[SIZE=1][COLOR=Yellow][U]Virtue Heroes (Serenity's Children):[/U] [B]@Eek a Mouse, The Devil's Mark, Outlaw Sniper, Gas-Soaked Rag Man, Amazon Prime, Friday's Child, Hot Blooded,[/B][B]Flower of the Moon[/B], [B]Rouge Demon Hunter[/B], Stimulated Emission, Animatronic Wench, [B]Lennie Small[/B]
[U]Virtue Villains (Serenity's Orphans):[/U][/COLOR][/SIZE][SIZE=1][COLOR=Yellow] [/COLOR][/SIZE] [SIZE=1][COLOR=Yellow] [B]Eek a Rat[/B], [B]Bomb Blondeshell[/B], Babe Brute, Jeanne Dark, Fallen Angle[/COLOR][/SIZE]
I routinely archive my chat logs, just so that folder doesn't get bloated. Do the same with my screenshots folder.
I just noticed that my CoH log folder weights around... 1Go, and some files are over 20Mo. Pretty big, isn't it?
Since 05-21-2008, my logchat is logging my "Fighting" tab (I don't know how it's named in english, but it's where damages, tohit chances etc... are shown).
Anyone knows how to remove this from the logchat? I have (and always has) 3 tabs: discussion, global, and "fighting" (or whatever it's called).