You're seeing just the callback
handle_write, go back to Membrane.Sink module for more information.
handle_write( pad :: Membrane.Pad.ref_t(), buffer :: Membrane.Buffer.t(), context :: Membrane.Element.CallbackContext.Write.t(), state :: Membrane.Element.state_t() ) :: Membrane.Element.Base.callback_return_t()
Callback that is called when buffer should be written by the sink. In contrast
handle_write_list/4, it is passed only a single buffer.
Called by default implementation of