The clusterer is used for combining closely positioned placemarks in a group (cluster).
The clusterer is implemented by the Clusterer class. The clusterer options can be passed to the constructor as parameters: the cluster icon display style, the size of its cell, and so on. For a list of options, see the corresponding section in the reference guide. Options for configuring cluster placemarks are set in the clusterer with the "cluster" prefix. For a list of cluster placemark options, see the documentation for ClusterPlacemark.
You can set a custom image for cluster placemarks. To do this, override the clusterIcons option.