Please find my brief answers below:
a) STG_LOSS represents the fraction of the storage loss in proportion to the initial amount stored, if the storage time would be one year. In other words, if 1 unit would be stored for one year's time, the amount lost would be STG_LOSS. If the actual storage time is shorter, the loss is calculated accordingly (assuming exponential decay the losses increasing either directly proportional to the storage time or according to exponential decay). The loss is always measured in terms of the stored commodity, an so it is not necessarily an energy loss.
b) STG_CHRG is an exogenous charge, which can be useful for inter-period storage, to set the initial level of the stock at the beginning of the horizon. It can also be used for timeslice storage, but I don't have any real life examples for that.
c) RS_STGPRD(r,s) is not an input parameter, but an internal one. It represents the amount of storage cycles under the parent timeslice of s. For example, DAYNITE storage is assumed to operate in daily cycles, and so the amount of storage cycles is the number of days under the parent timeslice. The number of storage cycles also defines the relationship between the storage capacity and the maximum nominal level of the activity variable, when the capacity represents the maximum nominal amount that can be stored.