You pick 100G on distance and on the number of fibers. DR1 is for 500 m of single-mode within the data center. LR4 covers 10 km, ER4 40 km and the ER4-plus variant 50 km; ZR4 goes up to 80 km. For those longer runs you use two fibers with duplex LC.
Only one fiber? There's a 100G BiDi for 20 or 40 km, with a simplex LC connector. You order it as a U/D pair, and the price is per module. Within the rack, a DAC cable is often the cheapest route. We code every optic for your platform.