The system will, for example, only show people in your guild when you are typing /gpromote. Names will also never be removed.
Changing this setting
In-game, type the following
/console autoCompleteUseContext setting
Alternatively, you can exit wow, open the Character's Config-cache.wtf file, and add the following line:
SET autoCompleteUseContext setting
Patch 3.3.2 (2010-02-01): Added