Reliable and efficient computation of cases with both attached BLs and large-scale separated flows present would require different methods in the different zones since large-scale separation is not easily described by RANS modelling and LES of external attached boundary layers could be too computational expensive. When unifying these two regions into the same simulation with a unified model formulation, there will be interface regions where the degree of resolution is varying. Fundamental problems are associated to these regions leading to well-known problems like the grey-area problem, log-layer mismatch, et.c. These RANS-LES interface regions with variable resolution is the focus of this paper. When turbulent flow is passing through regions of varying resolution (VR), the filter with will change. Consequently, the spectral cut-off wave number will change following the mean-flow. This implies that the energy cascade must be complemented with an additional and different mechanism of transferring energy between resolved and unresolved scales directly related to the rate of change of cut-off, which can be related to (i) the LES filter, (ii) the hybrid RANS-LES length scale, or (iii) the PANS ratio of unresolved turbulence energy. The rate of energy transfer can be precisely quantified, and the corresponding terms can be derived. In the paper, we will use the PANS framework (Girimaji 2006) for illustrating these terms following Girimaji & Wallin (2013). Furthermore, the concept has been extended to hybrid RANS/LES based on the K-omega baseline model for the unresolved turbulence (Carlsson et al. 2022). The final paper and presentation will show some progress on scale-resolved simulations including the modelling of the additional terms emanating from the variable resolution in space for some practical applications.