The DS works as a core part of a network operating system (NOS). Within a NOS, a directory service is an application or database.
A directory service needs to accept changes to the directory database.
If the database is not changed then the directory is useless.
Within the operating parameters of a network, the 'updated master directory' must be replicated.
The 'master...' must be replicated (available) to any other domain controller.
Thus, any domain controller accepts & replicates a change to the directory from any other domain controller.
This differs from the replication operations in other networks. In other networks, one computer stores the one 'master...' of the directory and the other computers store backup copies of the 'master...' .