Fractional variability (F_var) versus median flux density plots. The panels are as follows: (i) Merged light curves. (ii) CRTS only. (iii) ATLAS only. (iv) ZTF only. The term “UL” refers to 3σ upper limits of F_var. The red hatched areas in (ii) and (iv) represent the low flux density and low variability regimes to determine non-variable light curves. The cyan hatched box is typically occupied by WDs and is another non-variable regime.