Considered by Halo Wars's Lead Designer, Greame Devine, as probably the most important menu the the game. So large that the menu itself takes up the whole screen. It is used from, to quote the L.D, "... for everything. To building infantry, to building vehicles, to building buildings and for calling in Leader Powers" The player can navigate through the Circle Menu via the Analog Joystick, to select whatever they so choose
Source
http://forums.halowars.com/forums/1/91352/ShowThread.aspx#91352