  With regard to generic two-component systems, the theory of first variations
of global quantities is reviewed and explicit expressions are inferred for
subsystem potential energies and potential-energy tensors. Performing a
conceptual experiment, a physical interpretation of subsystem potential
energies and potential-energy tensors is discussed. Subsystem tidal radii are
defined by requiring an unbound component in absence of the other one. To this
respect, a few guidance examples are presented as: (i) an embedding and an
embedded homogeneous sphere; (ii) an embedding and an embedded truncated,
singular isothermal sphere where related centres are sufficiently distant;
(iii) a homogeneous sphere and a Roche system i.e. a mass point surrounded by a
vanishing atmosphere. The results are discussed and compared with the findings
of earlier investigations.
