It uses cmd.exe style auto completion. This means for people used to tuxxy auto completion it seems weird.
[Spoiler]In the Windows default shell, TAB shows you the next possible completion. If there is another string with the same prefix, it might be shown as completion instead. To get to the other possible completions, you have to cycle through them with TAB. ZKL additionally not only considers your entered string as prefix, but instead shows all possible completions containing your string as substring.
The engine also has built-in auto completion. It considers your string as prefix and will only add as many characters as possible while preserving the same set of completions. I.e. if there is Anteep and Anturf ingame, you enter An and hit TAB it will autocomplete to Ant. If you enter Ante + TAB it will autocomplete to Anteep. Engine autocompletion also works for unitdefs.
Kudos to
Licho for even implementing any kind of auto completion.