Board-Stacking Interconnect at 0.40mm Pitch
The Hirose BM20B(0.8)-24DS-0.4V(53) is a 24-position receptacle from the BM20 series, designed for parallel board-to-board stacking at a fixed 0.80mm mated height.
Position Count and Stack Height — Matching the BOM
The 0.80mm mated stack height is fixed; the board-to-board spacing in your enclosure must accommodate this height with no allowance for adjustment. Against the 20-position sibling BM20B(0.8)-20DS-0.4V(53), the difference is purely positional — the footprint pad layout changes, but the pitch and stack height are identical, so the board routing density stays the same.