The Hirose BM23PF0.8-40DP-0.35V(895) is a 40-position header from the BM23 series, built for board-to-board mezzanine stacking at a fixed 0.8mm mated height. The outer shroud contacts provide a self-aligning lead-in during mating, which helps when two boards come together with less than a millimetre of clearance to work with.
How the 0.8mm stack height drives the board layout
The 0.8mm mated stacking height is the single dimension that decides whether this connector fits your assembly. That is the total gap between the two PCBs when mated — the height of the header above its own board (0.025" / 0.63mm) plus the receptacle height on the opposite board plus any clearance for components underneath. At this height, the wipe has to seat at the worst case; there is no room for a board that bows or a solder fillet that stands proud.
RoHS compliant per current directive.