A trading cluster is a gathering of small traction towns and villages where trading ocurrs. During a trading cluster, the communities involved don't tend to 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. Another reason could be because 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 traktionstadtgessellshaft 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.