Graded seals are for connecting borosilicate glass components to metal. The metal (Kovar) length is approximately 25 mm while the borosilicate tubing is approximately 100 mm long. Available in 1/8, 1/4, 3/8, and 1/2" outer diameters.
Normally, one can not join metal directly to glass because the coefficients of expansion are different and the glass does not "wet" the seal well. A graded seal starts with a special glass formulation to make the initial metal-glass connection, and that is then connected to a series of short segments with slightly different coefficients of expansion until the glass at the end is pure borosilicate (like that used in all of our glassware). Thus, your glassblower can connect this piece to whatever glass apparatus or joint you wish.