Board layout fit and density
The 104666-3: This is a 40-position, dual-row header on a 1.27 mm pitch — 0.050" centerline — with a row spacing of 0.100" (2.54 mm). That pitch packs 20 contacts per row into a footprint that fits a standard through-hole grid; the 0.145" (3.68 mm) post length clears a typical 1.6 mm PCB with room for a solder fillet.
Signal-level rating and temperature envelope
Rated 30 VAC, which puts this squarely in signal-level duty — not for power distribution.
Mechanical retention and polarization
The shrouded 2-wall design with a latch holder gives this header a positive lock against vibration-induced disconnect — the latch engages the mating receptacle's ramp and holds until you squeeze the release. The 2-wall shroud also polarizes the mate, so you can't plug the receptacle in backwards.