In recent years different solitary waves, like solitons, vortex rings, solitonic vortices, and more complicated, were predicted. They can move in superfluid ultracold gases along elongated traps. A theoretical description of the moving demands knowledge of two functions, the inertial mass of the soliton and the effective number of particles. They can be calculated on the basis of a microscopic theory. However, one can express them in the the terms of observable quantities - the phase increment and the depletion of number of particles in soliton. In this note a transparent derivation of the corresponding equations is suggested. The results are applied to the "magnetic soliton" in mixtures of Bose-gases in different spin states,which was predicted recently.