When we teach, and even communicate with people whom we assume to belong to some sort of common circle with ourselves, we simply assume that our audience knows or at least have heard about certain things.
For example, with people belonging in some sense to physics I would expect that my physics students would at least know about Bessel functions. I was wrong. One has heard the name, the others had not any idea. And they did not know practically how to plot functions on a computer. Two did know sort of, but only using "Origin". And so on. I do not want to complain. They are bright young people and they know lots of things, but not
all what I - and most of my colleagues - would expect the students to know.