What Are the Requirements for an SMN Topic Name?
A topic name can contain 1 to 255 characters, including letters, digits, hyphens (-), and underscores (_). Each topic name in a user account must be unique. After a topic is created, SMN generates a topic URN composed of the service name (SMN), region name, project ID, and topic name to uniquely identify the topic. When you call an API to create a topic, a topic URN will be returned. A URN will be used whenever a publisher or subscriber performs operations to a topic.