Ive just added a picture to the first post in the above link, showing the pcd and centerbore. You can see that the center bore locates over the spigot on the hub, to centralise the wheel... too small and it wont fit. Too big and it would be loose and getting the wheel central would be tricky and there would be higher loadings on the bolts. Spigot rings can be used to reduce center bores, especially on mass fit aftermarket wheels.
Pitch Circle Diameter. usually written as (no of bolts X diameter in mm of the circle they are arranged about) 4x100 for Twingo hubs