It's used to define the dialog of a conversational bot, to be generated by the Chatbot generator.
In the New object dialog, it's under the Chatbots category:

Note that it contains two Tabs: Design and Tree inside the Instance Data.

The default ConversationalFlow instance looks as follows (Designer tab):

The main element of the Conversational Flows instance structure is the Flow.
You can also select the Tree tab that looks as follows:

HowTo: Build a chatbot using GeneXus
Chatbots in GeneXus