[boost] [asio] boost::asio::placeholders::error vs _1

What is the rationale for boost::asio::placeholders::error and boost::asio::placeholders::bytes_transfered? acceptor_ptr->async_accept ( *socket_ptr , boost::bind( &handle_accept , acceptor_ptr , socket_ptr , boost::asio::placeholders::error ) ); I find this a foreign and confusing usage a boost::bind. Granted I jumped over the Tutorial explicitly introducing these, instead starting with the Tutorial DayTime.3 that addressed my interests. I see the familiar boost::bind, with the lack of _1, _2, so I'm thinking bind is generating a function of type void ()(). ... but handle_accept is expecting a boost::asio::error&??? Thanks, Jeff