The presence of any attribute on a normal variable magically transforms that variable into a parameter which is read-only. This means that it can be used to pass data out of the model during simulation.
Such variables are initialized to the starting value at the beginning of the simulation only. Regular variables are initialized on every pass through the equation set, which occurrs on every Newton iteration. The read-only parameters can therefor retain history from the last iteration.
In WRspice, data from these (and all) parameters can be obtained from the @device[parmname] special vector construct.