Requests that the host informs the client about existing nodes of a given set of types.
The request is "sticky" like a subscription; the host will remember the mask, and keep sending node create/destroy commands to the client until it disconnects or changes the mask.
This is the standard way for a client to learn about the contents of a host, and a very common first command to send over a new connection.
Note that although this command name includes the word "subscribe", there is no corresponding "unsubscribe" command. Instead, use a different mask to stop subscribing to information about a given node type.