It was developed for internal use (for integration with existing customer system) - it allows you to subscribe for node/container events and data collection values.
I think that REST API for subscription management is not public right now.

update: actually you can manage it using java API (using nxshell, for example):

public void updateZmqEventSubscription(long objectId, long dciId, boolean subscribe) throws IOException, NXCException;
public void updateZmqDataSubscription(long objectId, long dciId, boolean subscribe) throws IOException, NXCException;
public List<ZmqSubscription> getZmqSubscriptions(ZmqSubscriptionType type) throws IOException, NXCException;

You need to set "ZeroMQEndpoint" in server's configuration (standard ZeroMQ notation, server use ZMQ_PUSH socket for communications).