Specific prior knowledge refers to those things that a learner has learned that are directly related (prerequisites) to a higher-order learning task which we have in mind. We we are talking about these learnings as “prerequisites” they are things that we say you need to know or be able to do in order to be able to do something else, not speaking to whether you already know them or not. Specific prior knowledge is those prerequisites to a learning task that somebody already possesses.
General characteristics (e.g., reading level, SES, regional affiliation, etc.) also relate to instruction for a particular topic, influencing sometimes how a topic is approached for some groups learners differently than for others or influencing instructional message design or strategy decisions for all learners.
The primary difference between the two is that specific prior knowledge determines what needs to be taught and general characteristics have an influence on how things should be taught.