The Hirose BM24-20DP/2-0.35V(53) is a 22-position header (20 signal + 2 power contacts) from the BM24 series, designed for board-to-board mezzanine stacking at a mated height of 0.8 mm.
Position Count and the Power Pair
The 22 positions break down as 20 signal contacts plus 2 dedicated power contacts — the "/2" in the order code flags that power pair. That means the BOM line needs to account for all 22 positions, not 20. The power contacts are physically larger within the same 0.35 mm pitch envelope, so the footprint has wider pads for those two positions. If you're laying out a board that only needs 20 signal lines, you still populate all 22 pads — the two power positions can carry a local supply rail or be left unpopulated, but the footprint is fixed.
Stacking Height and Board Spacing
The mated stacking height is 0.8 mm. The header sits 0.026" (0.65 mm) above its board.
If the BOM calls for more signal lines, the BM24-24DP/2-0.35V(53) is the direct step-up: 26 positions (24 signal + 2 power) at the same 0.014" (0.35 mm) pitch, same 0.026" (0.65 mm) height above board, and same 0.8 mm stacking height. No change to the routing channel width or the keep-out zone on the mating side.