Topic Name, while it is true that the Data Distribution Service (DDS) API specification does not mention a limit for Topic names, the complementary DDS Wire Protocol specification: Real-time Publish-Subscribe (RTPS) protocol (http://www.omg.org/spec/DDSI-RTPS/2.2) does state that Topic names shall not exceed 256 characters (see Table 9.12).
So the 256 character limit on Topic name lengths imposed by the Vendor: Real-Time Innovations (RTI) DDS implementation is not arbitrary. It is precisely what is required to be interoperable with other DDS implementations.
Source: Topic Name