Cube series are really complicated movie series.
At the end of the Hypercube, Kate gives a device some serious guys (some military guys maybe). This device was taken from Alex Trusks' neck.
What is the purpose of this device?
It was some sort of storage device with confidential information on it. I don't remember many other details though as to why she was looking for it or who had it initially.
The only information in the original version of the film is the last line spoken by an unknown African American in a suit, who presumably works for Izon or the military:
The fact he said "recorded on it" implies it's some kind of sensor device, such as a camera, microphone or other more high-tech information gathering system rather than a storage device, as usually you'd say "saved on it" or "stored on it" in that case.
There's more information given in the alternate ending which I can't seem to find, though Wikipedia says:
The fact that this was an experiment would seem to back up the idea that it was a sensor as the scientists working on the project would need a way to get data back from the experiment and there was no other machinery shown in the "real world" which could be monitoring what happened inside: