In North America, this refers to a minority- or women-owned staffing supplier. Organizations often find that using diversity suppliers as part of their staffing supplier base is a good way to meet their diversity recruitment goals. (See also: Women and Minority Business Enterprise). In Europe, a Diversity Supplier could be taken to mean a supplier with an equal opportunities policy in place, which ensures equal treatment for temporary workers from disadvantaged minority groups.
Source: Staffing Industry Analysts