From www.leather-dictionary.com - The Leather Dictionary
Most people assume that saddle leather is leather for horse saddles. But saddle leather is not a standardized term. While it is known for horse saddles, furniture, bags, belts and other objects are also made of saddle leather.
Nowadays, a lot of types of firm leather or leather with a patina look (vintage leather or oiled or waxed pull-up leather) is offered as saddle leather.
Therefore, it always helps to ask and identify what kind of leather is used when the term saddle leather is described.