Experimental data on diffusion combustion of round hydrogen microjets with a parabolic and a "top-hat" mean velocity profiles at the nozzle exit at different spatial orientation of the microjets are reported. Most of all, we are interested in the behavior of the so-called << bottleneck flame region >> of the jet and its contribution to the diffusion combustion. As is found, in the cases of the jet velocity opposite and orthogonal to the direction of the gravitational force, the main features of combustion are practically the same. Otherwise, at the velocity vector matching the direction of the gravitational force, the combustion characteristics become much different. Combustion in the << bottleneck flame region >> is found to be more stable at the parabolic profile while the stability of combustion is reduced at the top-hat velocity distribution at the nozzle exit. Then, the flame detachment occurs in the absence of the << bottleneck flame region >> and the microjet combustion is terminated at a much higher velocity. An inversion of the dependence l/d = f(U-0) is observed at the transition from the parabolic to the top-hat velocity profile of the jet. The ratio of the << bottleneck flame region >> size (l) to the nozzle exit diameter (d) is l/d. The nozzle heating is shown to have a profound effect on the microjet combustion.