According to Dave Hewitt,
"I describe something as arbitrary is someone could only come to know it to be true by being informed of it by some external means - whether by a teacher, a book, the internet, etc. if something is arbitrary, then it is arbitrary for all learners, and needs to be memorized to be known."
In other words, when things are "arbitrary", it means that something has been informed by someone or something, and there are no questions or opinions to this piece of information. Whatever is informed is meant to be true and without any doubts.
Hewitt continued to explain that "all students need to be informed of the arbitrary. However, the necessary is dependent upon the awareness students already have." In "necessary", students will take the "arbitrary" part and work out the properties that are in learning. "Necessary" requires students to discover their learning.
In planning lessons and units, teachers need to effectively utilize and distribute the "arbitrary" and the "necessary" aspects of teaching. There are learning points in which students have to memorize (arbitrary) and there are learning points in which students have to develop their thinkings from what is being informed (necessary). In designing lessons, teachers need to make sure that they are not teaching students that they have to memorize everything being told. Instead, there should learning activities that show students' thinking processes, and that they can develop and derive new information from the arbitrary.