But the lite-c workshop number 4 says that it is a flag for a panel
Well, then I'm sorry, the article in the manual isn't precise enough.
If this flag is set for a map or model, black or transparent parts of the texture become completely invisible.
I always use .png for that stuff.
especially because I had some problems with .tga files being mirrored horizontally
(image flipped because acknex ignores the 0,0 origin pos)
...and .pngs are a bit smaller ;P
EDIT:
that beeing said, why don't you simply use a .png, .tga, .dds or whatever and use an alphachannel?
That's much easier to handle and you'll have a soft alpha channel in stead of that ugly 1-bit alpha...