Clusters are used in buildings and can be filled with mechminds of any type and rating. The capacity of clusters variies by the importance of the different buildings. After filling it to it's capacity, the clan gains control over the building, increasing its influence in the sector where the building is located.
A clan can also lose control over a building when the cluster dissolves, which can happen when the building's production stops, or if the clan currently isn't having enough convinced mechminds in the sector.
The cluster obviously provides many benefits. The main one is that when all buildings have good production rate, all free mechminds brought into the building get automaticaly convinced for free. Another benefit is increased influence of the clan that captured the building. It also provides a spawn point for friendly mechminds, and if it is inside a base, then newly convinced mechminds can also spawn there.
Capturing a building does not provide better trading rates, or equipment discounts, it also does not change building protection turrets, they stay neutral