From the Wikipedia page 
Parkstad Limburg is a conurbation of eight municipalities in the southern part of the province of Limburg in the Netherlands. Consisting Heerlen, Kerkrade, Landgraaf, Brunssum, Simpelveld, Voerendaal, Nuth and Onderbanken, the municipalities work together to improve public services, transport and housing on a regional level. This collaboration started in 1999. Within Parkstad, the city of Heerlen functions as the centre of economic and social activity, and to a lesser extent the cities Kerkrade and Landgraaf function in the same regard as regional hubs.