Clarify Callback function arguments

[jrwrigh]

In the documentation for the solver callback function, it simply states

The callback function callback is a function which is called after every optimizer step. Its signature is:

callback = (params, loss_val, other_args) -> false

where params and loss_val are the current parameters and loss/objective value in the optimization loop...

This would leave me to believe that loss_val is the objective function value given the params. But in my testing, this is not true. Are the params the parameters for the lowest-found objective value? If so, what is loss_val then?