Different kinds of solitons can form bound states with each other and with strings. For example, a particle can lie on top of a membrane and, moreover, strings can stretch between the particle and the membrane. Black holes identified with certain bound states have a proper surface area that is not greatly affected by the vibrations of the solitons and strings. Bound states of solitons and strings agree better with intuitive notions of black holes than simple strings or isolated solitons. Specifically, they give a definite prediction for the entropy.
Bound states have an internal structure that is more involved than that of a single string.
The important qualitative feature remains that the microscopic structure is extremely small so the macroscopic observer is sensitive only to some bulk features, such as the mass. This is in perfect analogy with ordinary thermodynamic systems, where the molecular structure is invisible to macroscopic observers. In the case of heavy bound states of solitonic membranes and particles, with strings attached, the membranes can be considered to consist of a large number of coincident fundamental membranes, each endowed with a certain minimal mass. Similarly, the particles are really many coincident particles. The strings that connect membranes with particles can attach to any one of the fundamental membranes and particles.