  A variety of solitary waves, such as solitons, vortex rings, solitonic
vortices, and more complex entities, have recently been predicted to exist.
They can move in superfluid ultracold gases along elongated traps. The
theoretical description of this motion requires knowledge of the inertial
soliton mass and the effective number of particles in it as functions of the
soliton energy. While these functions can be calculated by a microscopic
theory, it is also possible to express them directly in terms of observable
quantities, such as the order parameter phase jump and the particle number
depletion in the soliton. In this article, the corresponding equations are
derived in a simple and physically clear way and applied to the recently
predicted `magnetic soliton' in mixtures of Bose gases in various spin states.
