X-ray luminosity as a function of emission-measure-weighted thermal energy kT_av (left panel) and the two temperatures and emission measures (right panel). The circles represent the synthetic spectra extracted from EMDs that are composed of time-averaged COs varying from a coverage of 10–100% of the total coronal surface (change of colour shade) and FLs – from Class C to X time-averaged over their evolution – that vary from 0 to 4% of the area of the COs (change of symbol size). The best-fitting parameters of the observations of Kepler-63 are overplotted with the blue square (2014) and the magenta circle (2019). In the background, the occurrences of each best-fitting parameter – that is, simulated 1000 times for each combination of magnetic structures by introducing Poisson noise – are shown with the corresponding colour bar in log scale.