What it is and what it does
The 3RT2637-1NF35 is a Siemens SIRIUS 3RT2 contactor — the workhorse for switching motor loads and resistive loads in control panels. It's rated 6 A at 230 V, which means it can handle a standard single-phase motor or a bank of heaters up to that current, and it's good for 3 A at 400 V if you're switching a three-phase load. Coil is AC/DC, 83–155 V, so it'll pull in on whatever control voltage you've got in that range without needing a separate coil rectifier.
Mounting and panel fit
Snaps onto a 35 mm DIN rail per EN 50022 — the standard stuff. 65 mm wide, 114 mm tall, 130 mm deep, so it fits in a standard 8-module wide enclosure with room for a few aux blocks alongside. Mounting position is flexible: you can rotate it ±180° on a vertical surface or tilt it ±22.5° forward/backward, which helps when you're shoehorning it into a tight retrofit panel.
Termination details
Power terminals are screw-type, accepting solid conductors from 0.5 mm² up to 4 mm² (2x each), and stranded from 10 mm² up to 50 mm² (1x). That's enough for a 40 °C rated lug on the main circuit. Coil terminals are also screw-type, so you're not hunting for a special tool on site.