Personal temporal abstraction
Pu means: Everything I don't know or understand at this moment.
Pu is an abstraction, comparable to R in math.
It's like a mathematical set of all mental components you classify as "don't know or don't understand". It has no counterpart in the real world, it only exists in your mind.
The contents of the mathematical R are the same for everyone. Sometimes hard to understand, but mathematicians can talk about it and create prove what it contains and should contain. It's at least inter-subjective, if not objective information.
The contents of pu are personal. First, you have to create pu in your consciousness before you can use it (that is still comparable to R). But what you place in pu is probably different then what someone else places there. Some people may start to place their big philosophical questions in pu:
-the meaning of life
-the core substance of consciousness
Other people would start to place their personal doubts in pu:
-should I start looking for a different job
-how can I go on in this relationship.
It's up to you. And that is very different to R. Pu is a personal abstraction.
Pu can be used in science to visualize the boundaries of what is known in science and what not. If scientists agree that a specific matter in their field of subject is pu, then pu becomes a bit more then personal, it becomes inter subjective. But pu is rooted in the personal usage.
The contents of pu can (and will) change over time. I can not speak Mandarin today. The Mandarin language is completely pu for me. If I do a course in Mandarin, then some parts of the Mandarin language fall out of pu into the "known"-parts of my brain. I don't know what will happen to me tomorrow, pu. But after tomorrow I will know what happened that day, not pu anymore. Pu changes over time, it is a changing set in your mind (very different from R). It is a personal, temporal abstraction.
Pu makes visible what you do not know, and that causes everything you do know in a new perspective.