The diagram uses the ArchiMate notation for physical elements, which are used to model physical concepts like machines, physical installations, materials, and distribution networks. A is an equipment element, which represents a physical machine, tool, or instrument that can create, use, store, move, or transform materials. B is a material element, which represents tangible physical matter or energy. C is a distribution network element, which represents a physical network used to transport materials or energy.
References: ArchiMate Physical elements1; ArchiMate 3.2 Specification, Chapter 9: Physical Elements2; Introduction to ArchiMate 3.0 (Part 4 Physical Elements)3.