Unisection refers to the process of finding what "things" (objects, expressions etc.) have in common. By finding out what they have in common with one another, you discover the medium from which they emerge as separate objects. In the CTMU, this common medium is called the unisect of the objects. If you start anywhere and start "unisecting" you'll eventually come to the most general elements of perception that you know e.g. space, time etc. The ultimate relation then between all these things is Reality itself.
The process of unisecting is really the process of abstracting.