Normally there are five different grades of leather and the details are as follows:

1)Full grain leather

2)Top grain leather

3)Split grain leather

4)Genuine leather

5)Bonded leather

Genuine and bonded leather aren't technically leather grades but the terms have been popularized within the leather industry, so we'll discuss them further to explain the complete leather grade spectrum.

All grades of leather undergo some form of treatment and tanning.

Before treatment and tanning, the hide is split, and this step is crucial to understanding how the different leather grades relate to each other.

The cowhide (also know as rawhide) ranges in thickness between 6 mm to 10 mm. Obviously, in the rawhide state, it not useable to make leather goods.

So the hide is split. The exact thickness of the split is determined by the customer's order and only takes the top cut into consideration.

The bottom cut has no bearing on the split. 

If a customer's order was for full grain leather that was ~1.4 mm thick, the split would be greater than 1.4 mm, probably closer to 3 mm, as the top cut will be further processed and compressed to create the final end product.

The top cut is the most valuable cut and contains the full grain of the hide.

The bottom cut is the left overs and quite often is split further.