Partially premixed combustion is one of the common combustion modes for many practical combustion systems where the mixture is classified as nonhomogeneous. The available experimental data show that the nature of mixing affects significantly the flame stability and structure of partially premixed flames at the same level of turbulence and local stoichiometry. Quantitative description of the mixing field is not yet available in order to discuss and analyze the effect of mixing on the flame structure and stability. The broad range of fluctuations in mixture fraction space within this mode requires proper classification into sub-ranges with common characteristics. This classification should be applicable for both laminar and turbulent flames. Accordingly a new diagram classifying the mixing field of partially premixed combustion into eight regimes within and partially within the flammability limits is proposed. The local mean and range of mixture fraction fluctuations are used to construct this diagram. One of those regimes covers the triple flames where the fluctuations lie within the flammability limits. The link between the stability and the proposed regimes is investigated using quantitative experimental data of turbulent flames. The data show promising correlation between stability and location of the flames within the diagram. The proposed regime diagram can be used as a platform for the classification of the mixing field of partially premixed flames.