The HBT method is used to get information about the sizes, shapes and sometimes also about the time evolution of the homogeneity regions in hadroproduction processes. Homogeneity region K is the region, where the hadrons with momentum K are produced. The shape and size of homogeneity region K is described by the Wigner function W(K,X) evaluated in the interaction representation after all the hadrons had been produced. Additional information about the evolution in time is contained in the emission function S(K,X). A theorem is presented and discussed which specifies which of the parameters characterizing the Wigner function can and which cannot be measured using the HBT method. In order to obtain the complete Wigner function additional assumptions are needed. For instance, it is enough to know the distribution of the centers of the homogeneity regions \langle X \rangle _K. In order to find the emission function further assumptions are required. No systematic analysis is available, but some instructive examples are discussed.

PACS numbers: 25.75.Gz, 13.65.+i