Using the Chuang and Nielsen Quantum Computation and Quantum Information book, for an introductory course in Quantum Information, I stumbled on an ambiguous (at least for me) use of the Von Neumann entropy, being used sometimes for quantum state (for example $S(\rho)$) and sometimes for quantum systems:
Suppose distinct quantum systems A and B have a joint state ρAB. Then the joint entropy for the two systems satisfies the inequalities $$ S(A,B) \le S(A)+SA(B) $$
Sometimes it is said that $S(A)$ is the entropy of a state in the system $A$, but the example cited above seems not the case.
My question is, what is the meaning of the Von Neumann entropy of a system? Does it always is related to the entropy of a state in the aforementioned system or has it a different meaning?