The Structure Of Earthman, Come Home

James Blish's Earthman, Come Home IN Cities In Flight (London, 1981) collects four stories but is divided into nine chapters after its Prologue:

Utopia
Gort
The Rift
He
Murphy
The Jungle
Hern Six
IMT
Home

However, the nine chapters present four adventures:

the warring colony planets of Utopia and Gort are in the same planetary system;
the planet He is in the Rift;
the planets of Murphy and Hern Six and the Jungle of destitute Okie cities are in the Acolyte Cluster;
the city called Interstellar Master Traders is on the planet that the former Okies come to regard as their home.

Occasionally, the texts of Cities In Flight give interesting hints of galactography. We are told that, in Arm II (of the Milky Way), the law is enforced by the Earth police and the economy is supported by the Okie cities (p. 171). A Dirac emergency message to Earth police is sent from "...ACOLYTE CLUSTER CONDENSATION XIII ARM BETA..." (p. 394) The reply from "...BETA ARM COMMAND EARTH..." states "SQUADRON ASSIGNED YOUR CONDENSATION ON WAY." (p. 395)

Arm Beta must be the same as Arm II. Thus, the Earthman culture covers not the entire galaxy but one spiral arm. The Acolyte Cluster has a unified defence force. However, its full address is not "Acolyte Cluster, Arm Beta" but "Acolyte Cluster, Condensation XIII, Arm Beta." I have never heard of "Condensations" as parts of the galaxy. However, galaxies are composed of stars that condensed from gas. Some stars are gathered in clusters and perhaps several clusters condensed from a single cloud of gas with at least thirteen such clouds forming one spiral arm?