Replacement bulbs if your bulbs burn out or break. These are for vegetative growth and can also be used for flowering and fruiting.
Your system will come with 6400K. This is the colour of light. 6400K is mainly for vegetative growth, and has a white / blue colour to it.
You can also choose 2700K. This colour is to optimize blooming and flowering as well as fruiting. Changing out bulbs near the end of the plant life cycle can improve yield and produce fruit faster.