that would be the type of item on the menu, for example greyed out, a separator, etc..
the second parameter to AppendMenu
the most useful ones would be:
0x0000 normal entry
0x0001 greyed out
however the only things that implemented that specific extension were MCS and Rabbit, so it's probably not worth implementing (however it wouldn't hurt to get most support out there to encourage future implementation)
in case you're wondering, yes i did have to check the rabbit source to figure that one out... lol