NATS Streaming Server by default embeds a NATS server. That is, the Streaming server is not a server per-se, but instead, a client to a NATS Server.
It means that Streaming clients are not directly connected to the streaming server, but instead communicate with the streaming server through NATS Server.
This detail is important when it comes to Streaming clients connections to the Streaming server. Indeed, since there is no direct connection, the server knows if a client is connected based on heartbeats.
It is therefore strongly recommended for clients to close their connection when the application exits, otherwise the server will consider these clients connected (sending data, etc...) until it detects missing heartbeats.
The streaming server creates internal subscriptions on specific subjects to communicate with its clients and/or other servers.
Note that NATS clients and NATS Streaming clients cannot exchange data between each other. That is, if a streaming client publishes on foo, a NATS client subscribing on that same subject will not receive the messages. Streaming messages are NATS messages made of a protobuf. The streaming server is expected to send ACKs back to producers and receive ACKs from consumers. If messages were freely exchanged with the NATS clients, this would cause problems.