what it does:
allows you to distinctly set the number of decimals for any prices / monetary values.

for most currencies, this is unnecessary as you can choose – in layout – to either display decimals or not (i.e 2.54 or just 2). However, there are some – though very few – currencies that typically use 3 decimals for which this constant is designed (in which case the value should be set to 3 ). However, if for example you wanted to use only one decimal for some reason, you could also set the value to 1.

affects:
number of decimals displayed for prices / monetary values

usable:
anytime, can also be changed or deleted again at any time.
however, changing/deleting this constant will not change values in already existing orders

example:

define('WPPIZZA_DECIMALS',3);/*integer >=0 */

where to add/define:

in your wp-config.php before:
/* That's all, stop editing! Happy blogging. */

Disclaimer:
As with all filters, actions and constants:
Although these should be safe to use as indicated, there are no guarantees. Use at your own risk. Make backups before usage.