Substantial means that the origin country /disembark country may produce halal leather and one cannot exempt the possibility of being paak leather. There are some places /counties you can not say that there is no chance of being halal like India. I mean if the leather has been imported from Kerlala, Tamil nado then there is probability of being halal but if it's from China then despite a percentage of Muslim population it's very hard to get satisfaction of being zabeehaa.