The quartz sleeve separates the ultraviolet lamp from the water while passing UV-C into it. Length and glass grade both identify a sleeve — length alone does not, and this catalogue stocks more than one sleeve at several of these lengths. This is the 200 mm sleeve for the B410. Note that this catalogue also lists a 410 mm sleeve covering the B410 under item 1007-0104 — the two are different positions on the same reactor family, so measure before ordering.