The Hirose DF17(3.0)-40DS-0.5V(57) is a 40-position receptacle with center strip contacts from the DF17 series, designed for board-to-board parallel stacking. The receptacle mates with the matching DF17(3.0)-40DP-0.5V plug; the two halves lock together at one of three mated stacking heights: 5mm, 5.5mm, or 7mm. That height range gives the layout engineer flexibility to fit the connector pair into different z-height budgets without changing the PCB footprint.
The three mated stacking heights — 5mm, 5.5mm, and 7mm — are the primary selection parameter for this connector. The 0.50mm pitch and 3.50mm height above board on the receptacle side are fixed; the stacking height variation comes from the plug side of the pair.