Each of the 13 federal circuit courts of appeals will get a Justice assigned to it, primarily to deal with one-Justice functions (for stays, extensions of time, and the like) coming from that Circuit.
They are sometimes given to Justices primarily based on their connections to the circuit. Several Justices have the circuits on which they exhausting served: Roberts (D.C.), Sotomayor (second), Alito (3d), Barrett (seventh), Gorsuch (tenth). Justice Thomas has the eleventh, presumably as a result of he is from Georgia. Naturally, some Justices must double up (and Chief Justice Roberts is quadrupling up, although three of his circuits are those with the smallest caseloads).