The Hirose DF12(4.0)-20DP-0.5V(86) is a 20-position header from the DF12 series, designed for board-to-board mezzanine stacking.
Key specs for the board layout
The 0.50mm pitch is tight — it drives a dense PCB footprint, so the trace routing between pads needs care to avoid crosstalk in a signal path. The 20 positions in two rows give a 10-per-row layout. The mated stacking height is 4mm, which sets the z-height between the two parallel boards; confirm that your enclosure or mezzanine standoffs match that dimension.
What it mates with
The outer shroud contacts on this header align with the receptacle's inner contacts. If you are stocking spares, stock both halves or neither — an orphan header without its socket is dead stock on the shelf.