Comparison plot of PF as a function of I derived from BISTRO observations in three different regions: NGC 6334 in orange (this work, similar to the middle panel of Fig. 8), NGC 1333 in blue (Doi et al. 2020), and Oph-A in black (presented in Kwon et al. 2018, and also Furuya et al., in prep.). The concentric filled circles on the bottom right of the plot indicate the relative sizes of the regions corresponding to about 10, 2, and 0.2 pc in total extent, for NGC 6334, NGC 1333, and Oph-A, respectively. The angularresolution of the BISTRO data are 0.09, 0.02, and 0.01 pc in physical scales for NGC 6334, NGC 1333, and Oph-A, at the distance of 1.3 kpc, 299 pc, and 150 pc, respectively.