Odd dimensions
An odd width such as 21 has one center column and one center block. This is useful for towers with a central pillar, compass alignment, and rotational designs that need a single origin.
Even dimensions
An even width such as 20 has a center line between two columns. A circle with equal even dimensions therefore has a two-by-two central area rather than one central block.
Which should you choose?
Choose based on the structure attached to the circle. The Odd center toggle raises an even input to the nearest supported odd size; disabling it preserves the exact dimension you entered.