I was just fiddling with buttons on my CI the other day (I was trying to find the sensitivity) when this "+L" came up. Then I pressed the same buttons again and a "-L" comes up. I did this a few times to try to work out what that meant but it all sounded the same.
Then I went to call Garth and I went to switch it to the telecoil and it wouldn't do anything, it just beeped at me. It was THEN I figured that +L and -L means lock!! It locked the controls! That'd be a safety thing for little kids. But I'm glad I worked that out :P