That's an exemption from the federal minimum wage (the "MW" next to it). It seems to be designed to encourage employers to hire disabled workers they might not otherwise employ, though I don't know whether it has positive effects in practice. It does have some provisions aimed at reducing abuse, such as requiring employers to first come up with an objective test of disability relative to the job in question (e.g. this person performs at 80% of the capacity of a typical non-disabled worker), then to determine the prevailing wage for non-disabled workers in similar jobs, and then to pay them no less than e.g. 0.8 x prevailing_wage, which can as a special exception to the minimum wage drop below the usual minimum-wage level.
PDF: http://www.dol.gov/whd/regs/compliance/whdfs39.pdf