Understanding data abstraction levels

The three-level ANSI SPARC Database Architecture suggests three data abstraction levels, namely, *external*, *conceptual*, and *internal* levels. If I understand correctly, the *external* level represents the users' views, the *conceptual* level is the conceptual diagram (ER model), and the *internal* level is the relational model. My problem is that I found in the literature that besides those, there is a fourth level (the lowest one), which is the *physical* level. I want to understand what is it concretely? Is it at this point that we define the type of index, access paths, and things related to physical access to data?