The study of two types of T-10 regime-a regime with low impurity confinement (S regime) and a regime with enhanced impurity confinement (B regime) has been continued in deuterium discharges with ne approximately necrit. In the present experiments the transitions from S to B regimes is obtained by the method of gas puff cut-off. Detailed investigations of the profile evolution of Te(r), Ti(r), ne(r) and carbon density nc showed, that the ne(r) profile greatly changes after gas puff cut-off, while Te(r) and Ti(r) remain practically unchanged at the central zone of the plasma. This indicates the possibility of a relation between the limiting ne(r) which is established and either Te(r) or Ti(r). It is shown, that the main cause which is responsible for this transition between B and S regime, is a variation of the neutral atom influx to the plasma. The transition itself takes place at a given plasma radius when ne reaches some limiting value at this radius.