In the application development manual, in the document gstreamer/docs/manual/basics-bus.xml, it says that "The return value of the handler should be TRUE to remove the message from the bus" when explaining what a message handler added with gst_bus_add_watch () does.

According to a comment in the code sample further down in the same document, and the documentation for both gst_bus_add_watch and GstBusFunc, the return value does not control whether the message should be removed from the bus or not, but rather whether the handler should receive more messages or not.