A single instance of a partition is called a replica. Partitions can have multiple replicas, but only one replica of a particular partition can exist on each server.
Servers can hold more than one replica, as long as each replica is of a different partition.
The initial partition replica is called the master replica. There is no restriction on the number of these replicas. A replica also stores a list of replica pointers that identifies all the replicas of that partition. The replica relies on the list of replica pointers to spread updates throughout the partition. Given this arrangement, a request to add an object to partition A could be sent to any of the servers.