It is the length from the bottom of the crotch to the bottom of the leg
It means that for pants of the same size, the leg of 35"inseam is 2 inches longer than the leg of 33"inseam
Bottoms Size refers to bottoms size, which corresponds to Tops Size.
My understanding is that Bottoms Size is the collective name for Waist Size and Hips Size.
But in actual application, it usually refers to waist circumference
Hope it is useful to you