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