Abbreviation: DF = "Dual Focus" (Sigma)

The abbreviations means "Dual Focus"

Explanation from Sigma

The DF (Dual Focus) system disengages the linkage between the internalfocusing mechanism and outer focusing ring when the focusing ring ismoved to the AF position. This system provides easy and precise handlingof the lens, since the focusing ring does not rotate during autofocusing. Thewide focusing ring also enables easy and accurate manual focusing.

See links

sigma-photo.co.jp