After the particle interaction with surfaces are recorded and digitized the hits on some detector elements need further treatment.
This struct serves as default structure of the process to take the digitized clusters on a detector element and provide the corresponding space point. The empty class is used to forbid the usage of an arbitrary data type as template parameter and enforces the implementation of explicit structures.
The choice of which kind of data should be treated in which way is steered by the choice of the template parameter. This parameter represents a structure that needs to store at least a cluster/multiple clusters and the corresponding space point.