DN stands for Digital Neo and it's used by Sigma to denote that lens is designed exclusively for mirrorless interchangeable lens cameras. Usually contained in lens name eg. (30mm F2.8 DN | A)
Posted by: Radovan 24.02.2013 21:32:25DG is abbreviation for DG for Digital and used by Sigma to denote that lens is designed for full-frame digital SLRs but it'll also work with ASP-C size DLSRs. Usually contained in lens name eg. (150mm F2.8 EX DG OS HSM APO Macro)
Posted by: Radovan 24.02.2013 21:21:12Abbreviation used by Sigma to describe special lenses designed for APS-C size sensors. Lens designated with DC are intended to be used only on cameras with ASP-C sensor. Usually contained in lens name eg. (18-50mm F2.8-4.5 DC OS HSM)
Source: