According to the ArchiMate language structure, a concept is either an element or a relationship. An element is either a behavior element, a structure element, a motivation element, or a composite element. A and B in the diagram correspond to these concepts respectively.
References: ArchiMate 3.2 Specification, Chapter 3: Language Structure, Section 3.2 Top-Level Language Structure; ArchiMate 3.0 Specification, Chapter 3: Language Structure, Section 3.2 Top-Level Language Structure.