The Borg Uni-Matrix

What exactly is a Borg uni-matrix? Is it really nothing more then a group of a few drones? Or are they much larger, extending through the collective so that each Borg vessel or outpost would have a number of drones from each uni-matrix? The latter of the two theories would seem to be more plausible. Uni-matrix one, which is supposed to function to serve the Borg Queen, appears to be scattered throughout the collective, as Seven of Nine was part of uni-matrix one, but was actually nowhere near the Queen. This would mean that the groupings into which the Borg are organized (resulting in names such as three of five and seven of nine) are not uni-matrices, but operational subunits, consisting of Borg drones that do not nessarlally belong to the same uni-martrix.