A liner is the inner lining of the riding helmet that is adapted to the size and shape of your head. Everything to get a comfortable helmet with the best fit.
The KEP riding helmets come in 2 different shell sizes and the sizes are adjusted with 2 kinds of liners, one for oval heads and one for round heads.
The most common is round, but if you usually get pressure in your forehead, an oval (long) shape is probably better for you.
M sizes are 52, 53, 54, 55, 56, 57, 58
L sizes are 59, 60, 61, 62