In the context of interface design, chat refers to the patterns and components that support the feature and interactions for either a real-live exchange between an agent (either a customer/technical support personnel or an automated bot) and the customer.
For questions specifically related to chat bots and automated interfaces and interactions the more specific tag 'chat bot' should be used. For questions that either involve a real person as the agent or can be applied both to a real person or chat bot then the 'chat' tag can be used.
