Requiring deep understanding of robot_state_publisher and TF

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Hi all,

I have turtlebot2, groovy, turtlebot arm. In reading the tutorial Using urdf with robot_state_publisher, I'm confused in its launch file.

Why there are two "state_publishers" ?

one is "robot_state_publisher", and the other is "state_publisher")

By saying "need a method for specifying what state the robot is in" the tutorial points out the function of "state_publisher",

But in checking robot_state_publisher package we can see the function of robot_state_publisher is "allows you to publish the state of a robot to tf", which seems similar to the function of "state_publisher".

My goal is to broadcaster TF info, so my turtlebot arm can do grasping based on it(which have tf listening function). I have a URDF of turtlebot with arm.

Thanks for your help!