The Hirose DF40TC-30DS-0.4V(51) is a 30-position receptacle with center strip contacts from the DF40T series, designed for board-to-board mezzanine stacking at a 1.5mm mated height.
At 0.40mm pitch, the board escape routing needs micro-vias or fine-line traces; this is not a pitch for a 2-layer prototype board. The 30-position count matches a 30-circuit signal bus; verify your BOM line count against this number before committing the footprint.
Gold flash at 1.97µin (0.050µm) is the lightest plating grade in the Hirose DF40 family — adequate for the moderate mating cycles of a device that gets assembled once and stays mated.