The overall aim of this article is to stimulate discussion about the activities within CER, and to develop a more thoughtful and explicit perspective on the different types of research activity within CER, and their relationships with each other. While theories may be the most valuable outputs of research to those wishing to apply them, for researchers themselves there are other kinds of contributions important to progress in the field. This is what relates it to the immediate subject of this special journal issue on theory in CER. We adopt as our criterion for value "contribution to knowledge". This article's main contributions are: (1) A set of 12 categories of contributions which together indicate the extent of this terrain of contributions to research; and (2) Leading into that is a collection of ideas and misconceptions which are drawn on in defining and motivating "ground rules", which are hints and guidance on the need for various often neglected categories. These are also helpful in justifying some additional categories which make the set as a whole more useful in combination. These are followed by some suggested uses for the categories, and a discussion assessing how the success of the article might be judged.