The model proposed to describe the possible two-stage superconducting phase transition in 2D (and quasi-2D) metallic systems is in fact very simplified in order to investigate their most typical and general features. All the more surprisingly that it catches some essential details which are characteristic for underdoped HTSC copper oxides. In particular, the experimental **data** demonstrates [28,29] that i) indeed for low n f the critical temperature T c is proportional to n f (what is simply ǫ F ), ii) T c shows saturation when n f approaches so called "optimal doping" (i.e. carrier concentration when T c as function of n f reaches its highest possible in given compound value), iii) the ratio T c /ǫ F in these and other "exotic"