Each ring is divided by radially directed lines into smaller nearly square cells. Their linear dimension is about 200pc. Disc galaxies do not rotate rigidly. The angular velocity decreases inversely with distance on moving away from the center. In the model this is implemented by allowing the rings to rotate at different angular speeds, which means a cell change its neighbours from time to time.
Neighbours means two cell share some length of common boundary.
Typically this gives a region 6 neighbours. Although 5 or 7 are also possible, we don't allow it in the model to simply the simulation. This is done by initialize the galaxy in such a way that all the cells have 6 neighbour, and when rotating the galaxy, a cell can only be moved an integer number of cell positions. This can simplify the model even further. The presence massive stars is the trigger of star formation.