TMAPI defines a basic set of features which may or may not be supported by an implementation. Some implementations may allow to turn some of these features on or off (although some may specify a default setting for a feature which cannot be altered). However, all implementations are required to recognize these feature strings and to be able to tell you what the current setting is (using the method TopicMapSystemFactory.getFeature(String)).
The current set of feature strings that all implementations are required to support can be found in the TMAPI Features Registry on the TMAPI website at http://tmapi.org/features. For convenience, those feature strings are described here.
Some implementations may define additional feature strings that are implementation-specific. Whenever you use a feature string that is not specified in the TMAPI Features Registry, you should always use TopicMapSystemFactory.hasFeature(String) to test if the factory you are using supports the feature before attempting to retrieve it with a getFeature() or set it with a setFeature() call.