The 0.156" (3.96mm) pitch is the standard SL-156 spacing — the same pitch used across the entire series, so the board footprint for the mating header is consistent regardless of position count. The 3-position count is the BOM line match: a 3-circuit harness needs exactly this housing. The polarizing key prevents mis-mating with a non-keyed header, which is a practical safeguard on a production line where multiple SL-156 connectors share a harness bundle. Crimp termination means the socket contacts are attached to the wire before insertion into the housing.
Typical TE part numbers for these contacts include the 770875 series (tin-plated) or 770876 series (gold-plated), depending on the current and cycle-life requirements of your application. The crimp terminals are inserted into the housing after wire termination; the locking ramp and housing geometry retain them.