I've rewritten the doc to the following:

Implementation of ContainerMatcher that returns a collection of containers to deploy a ModuleDescriptor to. This implementation examines the deployment manifest for a stream to determine the preferences for each individual module. The deployment manifest can (optionally) specify two preferences: group and count.
A group indicates that a module should only be deployed to a container that belongs to a group. If a group preference is not indicated, any container can deploy the module.

If a count for a module is not specified, by default one instance of that module will be deployed to one container. A count of 0 indicates that all containers in its specified group will deploy it. If no group is specified, all containers will deploy the module.

In cases where all containers are not deploying a module, an attempt at container round robin distribution for module deployments will be made (but not guaranteed).