I ask this, as the Traveller Map, the source of all stellar mappy goodness, purged what was considered non-canon material a short while ago (including a LOT of DGP material, dammit), meaning that I had to go find the Sunbane data for Yahehwe Sector (thankfully, a version of it's on the Traveller Wiki), and rebuild the map for it, so I could continue working on my alien module for the H'Hnaarouhw, which will eventually be a client race of the Aslan Heirate.
However, the original data on the Traveller Map, to the best of my recollection, included express communications (X-Boat) routes of some form, which don't, of course, appear in the Yahehwe Sunbane data.
Still and all, while this answers how the data came to be, it doesn't answer the question on how the Aslan go about deciding how their "X-boat" routes are worked out, though
Travellers Digest #15 has its routes page marked "Xboat Routes of Daibei Sector"
Travellers Digest #16's equivalent map for Reavers' Deep records that the routes are Xboat routes for the Imperium but trade routes beyond.
Travellers Digest #17's map is labelled "Trade Routes of the Dark Nebula Sector"
So the routes on TM are ostensibly trade routes, although it is likely they were selected to "look good" rather than necessarily follow the most likely trading routes as identified by the rules.
Travel and Trade Routes
The Hierate has nothing to correspond to the Imperial x-boat network and no travel routes are plotted on Hierate subsector maps. Major trade routes connecting high population worlds are often established by clans and clan trade routes may be plotted; they connect worlds with population 9 +, usually calling at established A and B starports along the way.
Individual clans maintain high-speed couriers that relay news and information much like an x-boat but there is no coordination or central authority. News travels across the Hierate as far and as fast as someone is willing to pay for it. - pg 153 Alien Module 1: Aslan (Mongnose Publishing)
. . .
As I see it, the Xboat network exists primarily as a holdover from the early days when the 3I was a fledgling trade alliance, . . .
Except that the X-Boat network was established by Empress Arbellatra in the 600's after the first two Frontier Wars and the Civil War, using for-the-time state-of-the-art TL13 Jump 4 technology.