A trading cluster is a gathering of small traction towns and villages where they typically trade goods and resources. During a trading cluster, the communities engage in a gentlemen's agreement to not partake in consumption of or eat each other because most of time they are the same or nearly the same size so it is impossible for them to devour each other. In addition, there's usually little time for them to trade as they have to do so before bigger towns and cities come across the cluster and preys on them.
The biggest trading cluster known to have ocurred during the series is during the preparation for the attack of the Traktionstadtsgesellschaft on the Green Storm, the biggest and most important of traction cities were present in the cluster such as Manchester, Monrou, and even Airhaven. It also consisted of hundeds of smaller cities, towns, suburbs, villages and hamlets. The cluster was for both the trade of goods and plans for the invasion on Shan Guo and its surrounding territories, it was also a way for the cities to gather for the attack without raising too much suspicion of the green storm.
Another trading cluster was seen in the film, where Salthook was gathering with other cities before being forced to move away when London began moving toward the cluster. It then ate Salthook, but the others presumably went away from the site.
Tom Natsworthy and Hester Shaw escaped London and eventually board a small town called Speedwell, where the owner Orme Wreyland attempted to harm them and schemed to sell the pair as slaves for profit at a trading cluster. Hester nearly managed to kill Wreyland, and, along with Tom, managed to escape from Wreyland, meeting a friendly airship pilot called Anna Fang, who takes them in her airship the Jenny Haniver to the neutral flying city of Airhaven, where they can find passage to London.