Quick answer - Tabs are created during the 'category' variable setting from the ulx.command object setup.
Long answer -
ULib is documented publicly here -
http://www.ulyssesmod.net/docsULX, which heavily uses ULib, commands aren't documented publicly, but are commented pretty well within the code itself. (And, even the code is in english, no obfuscated variable names or command structures)
Then, when ULX commands are set up correctly, they're automatically added to the tabs within XGUI.
I'd recommend looking over the TTT commands release by terrortownteam -
http://forums.ulyssesmod.net/index.php/topic,6279.0.htmlThey make thier own category and XGUI tabs mostly by just adding a new ulx.command ("Category" ... ) and using it for the rest of the functions.
If you have further questions needing clarification, feel free to ask.