The theoretical description of experimental data on transport in open systems has been of interest for more than a hundred years. Boltzmann proposed a new view of the transfer of matter. Now it is possible to describe the transfer processes from a microscopic point of view. The solution of the Boltzmann equation and the equations derived from it is a complex problem related to the mathematical problems of solving such equations. On the other hand, the complexity of the solution is related to the geometry of the system in which the transfer process takes place. Fundamental physical calculations are made for systems: flat, slotted, cylindrical, rectangular, etc. In the free molecular mode of gas flow, collisions of molecules occur mainly with the walls of systems. In this connection, there was a direction related to the calculation of the probabilities of atomic transport in the system. In this paper, we propose an approach for determining the probabilities of atomic outcomes from slit systems depending on the relative height of the walls of H systems. Exact formulas are obtained for calculating the probabilities of atomic departures from systems without colliding with walls, the distribution of atomic collisions over the height of the system wall, the probabilities of atoms entering the condensed phase after a single collision with the system walls, and the probabilities of atomic departures from systems after a single collision with walls. The accuracy of the obtained formulas was compared with the data obtained from computer experiments using the Monte Carlo method.