ASME standard B16.9 Butt Weld Pipe Fittings

https://www.pipefittingchn.com/bend-pipe/

LR 90 Elbow: Fittings that replace course in a piping approach are referred to as elbows. The directional difference is given in levels, reminiscent of 45 or 90. Long radius 90 measure elbow has the center line of elbow at 1.5 x NPS from elbow conclusion