Minecraft wiki entry for Ticking_area, associated text: In the first form, specify two pairs of coordinates in the world. The coordinates specify the opposite corners—either northeast and southwest or northwest and southeast—of the ticking area.
In the second form, specify a center coordinate and a radius of 1 to 4 chunks. Although these arguments suggest a circular area, in actuality the ticking area includes all chunks within 'r' chunks, including diagonally, of the chunk containing the center. This defines a square whose sides are twice the given radius + 1. For example, specifying a radius of 2 results in a square ticking area 5 chunks on a side.