DF40HC(4.0)-40DS-0.4V(51) from Hirose Electric Co Ltd is a rectangular board-to-board mezzanine connector used where compact interconnect density and reliable mating are important in embedded electronics. Within the Connectors, Interconnects > Rectangular Connectors category, this type of part is commonly selected for space-constrained designs such as industrial controls, portable equipment, display modules, communication hardware, and other multilayer PCB assemblies. The supplied product data indicates Active product status, which is favorable for ongoing procurement and new-design consideration, although engineers should still confirm long-term availability with the manufacturer or authorized channel. Moisture sensitivity level is listed as MSL 1 (Unlimited), supporting standard assembly handling expectations for many production environments. RoHS compliance information is not explicitly provided in the supplied specifications, so compliance status should be treated as information not available until verified through the official datasheet, manufacturer declaration, or distributor compliance documentation.