How to interrpit joint state data from gazebo [closed]

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I am writing a simple odometry for a two wheeled robot. I would like someone to confirm my understanding of of the joint state data from gazebo shown below.

set_point is the cmd vel sent to the wheel

process_value_dot is the angle part of the angular velocity, 0 means no rotation.

process_value is the position part,velocity translation along the principle axis.

process_value is the actual velocity translation movement of the motor.

error is the difference between the motor velocity and the cmd velocity sent. The error value can be caused by incorrect PID settings to overcome inertia and gravity.

The time stamp is generally sub sec so that between any two cmd msgs. Distance is traveled is (process_value_new-process_value_old)(new time - old time)

If set_point=0 but a process_value>0 then this is motor drift probably caused by PID setting.

What are these values?

seq: ???? can this be used in some way to associated left right motor states arrivals. time_step: ?????? command: ???????

header: 
  seq: 61
  stamp: 
    secs: 7
    nsecs: 98000000
  frame_id: ''
set_point: 0.0
process_value: 0.0183130319794
process_value_dot: 0.0
error: 0.0183130319794
time_step: 0.01
command: 0.82404868582
p: 45.0
i: 0.2
d: 0.1
i_clamp: 0.0