What is a topic hierarchy?
Many publish/subscribe message systems support a tree-organized topic arrangement, many of which predate JMS. For example, suppose the server in question supports topic hierarchies and it is managing the topic "Dog" and the subtopics "Kuvasz" and "Mastiff". In this case, a message published to "Kuvasz" and a message published to "Mastiff" either would be distributed to subscribers of "Dog", or not, depending on the characteristics of its publish/subscribe model and/or configuration parameters. That is, subscribing to "Dog" would, in effect, constitute a subscription to all subtopics such as "Kuvasz" and "Mastiff," if configured, for example, with a property setting such as "includeSubtopics=true".
The JMS specification does not require support for topic hierarchies. Although topic hierarchies can be a powerful tool, it's important to consider whether or not portability is an issue. Topic hierarchy designs vary across JMS servers, and not all JMS servers implement topic hierarchies.