The concept of assimilation is absolute. Technology on a nanometer scale is able to subvert a being and convert it to a member of a civilization to whom individuality is a meaningless word. The assimilated lose their identity, their "I" and "me" and "self"; now only "we" and "this drone" exists. The single Borg drone is relatively helpless, yet trillions together form the most dangerous threat to the Federation, if not the galaxy...or even universe.
However perfected a technology may be, there will always be glitches. Each being is unique in some minute way; and one-size-fits-all is definately not true. Use the technology trillions of times, and a failure rate of .0001 percent still leaves thousands of mistakes. When a problem crops up in assimilation which can not be compensated for, the result is known as assimilation imperfection.
The imperfectly assimilated of the Borg empire are normal drones in many ways. They believe in the quest towards perfection in Oneness, even if they are denied the extreme closeness of the Greater Consciousness. Technologically they rely on their implants, nanites, and subspace transcievers to remain tied to each other and their machines. What is not normal, however, are the shreds of individuality which remain.
Individuality detracts from the perfection of the Collective. Imperfectly assimilated drones attempt to be One, but their remaining individuality continues to crop up. For the Collective, it can be compared to having several voices questioning what is happening at the most inopertune times. To remedy the problem, Cube #347 was commisioned.
Exploratory-class Cube #347 is the dumping ground of imperfectly
assimilated drones. Kept together on one ship, their mental signatures
can be isolated from the Greater Consciousness in order to lower the
impact of their "individuality". The ship itself functions in the general
manner of a sub-collective, although it is generally given greater freedom
to solve its problems...perhaps on the hope that the problems will solve
Cube #347. The cube is sent out on long assignments of otherwise low
priority; t he sub-collective does have its uses, after all.
Unfortunately, when trouble brews (and even when it doesn't), the closely
knit members of the cube have the tendancy to fall apart and attempt to go
their own ways.