Owning 20 or more slaves--It allowed very wealthy men a way out of the war and became very controversial in the South.
Southern men who were not exempt from the war found this exemption insulting. The war was coined as a "rich man's war" being the war was being fought for a states' rights to allow slavery and then the slave holders were allowed to exempt fighting. Economically, this helped keep product for sale and bring money into the Confederacy.