  We consider four definitions of solution to the initial-boundary value
problem for a scalar balance laws in several space dimensions. These
definitions are generalised to the same most general framework and then
compared. The first aim of this paper is to detail differences and analogies
among them. We focus then on the ways the boundary conditions are fulfilled
according to each definition, providing also connections among these various
modes. The main result is the proof of the equivalence among the presented
definitions of solution.
