The carious lesion marked by the arrow represents:
Recurrent Caries:
Recurrent caries is that type which occurs in the. It is usually due to inadequate extension of the original restoration, which favors retention of debris, or to poor adaptation of the filling material to the cavity, which produces a ‘leaky margin’. 
Ref: Shafer's textbook of oral pathology 7th edition page 445
Forward caries
Backward caries
Recurrent caries
Smooth surface caries