In the equation GPP - R = NPP
R represents:
Radiant energy
Retardation factor
Environment factor
Respiration losses
GPP is gross primary productivity that is the rate of total production of biomass by producers.
R is the respiratiory loss. A part of biomass produced by the plant utilized by them for their respiration.
NPP is the balance of biomass or energy remaining after meeting the cost of respiration of producers.