Global and local edge parameters at the H-mode transition are investigated at edge densities below and near the H-mode density limit. Conditions for onset of edge magnetohydrodynamic (MHD) instability (ELMs) and sustaining the H-mode define an operational window for H-mode with a maximum edge density, which can only be reached with optimum adjustment of both heating and fuelling fluxes. Careful experiments are carried out to vary the edge density in a wide range and allow comparison with H-mode theories in various collisionality regimes.