Geography help a levelWatch
I would guess a rocky coastline is a coastline of destructive nature, with high energy waves over a long fetch, bands of hard rock with resistance to wave erosion to form promontories. Whereas a coastal plain is a submerged shelf beneath the sea. This would mean an area with constructive waves and a low energy coastline that allow the development of an offshore platform and sediment accretion and accumulation of cliff material due to rockfalls and landslides. Whereas on a high energy coastline, cliff material is eroded and washed out to sea.