Examples of theoretical fits to selected FeH lines and resulting distributions of filling factors for GJ 388 assuming RC (first and second rows) and MC (third and fourth rows) magnetic field models. Atmospheric parameters T_eff = 3400 K, α(Fe) = −4.64, υsini = 3 km s^-1. Dashed violet line: computation with zero magnetic field; red line: computation with multi-component magnetic field shown in the bottom right plot; blue line: computations with homogeneous magnetic field (i.e. f = 1) of the same intensity as in the multi-component magnetic field.