Short stay or long stay visa?

C visa - short stay (Schengen)

You wish to visit Belgium for a maximum of 90 days, generally valid for 6 months from your first arrival on the territory, e.g. family, friendly or professional visits, tourism, conference.

D visa - long stay (Belgium) 

You are going to settle permanently in Belgium or for more than 90 days at least, e.g. family reunion, studies, work.