The Scubapro Superflow 210cm Hose is a high airflow hose for your regulator with a large, smooth internal bore so it won't restrict air flow. The rubber outer shell both protects the inner hose and adds some weight to the hose so they don't float in the water.
3/8" male thread will fit most regulator 1st stages today (except some Mares and older Apeks that use a 1/2" thread but you can read more about thread sizes here) and the female end is the universal 9/16” that fit all second stages without a swivel or special connection.
The female end is fitted with a high torque nut that requires a special tool to tighten properly.
210CM
2.1m is usually the longest length hose divers use and it's most used by twinset divers for a long hose primary donate setup. 2.1m allows divers to donate gas and travel single file through an obstruction or restriction.