what it does:
there are 3 different constants you can use to not install default items/categories and/or pages
(see below examples for details)

affects:
frontend/backend – on first installation only (also on first activation on child sites in multisite setups)

does not affect:
does not do anything after the plugin has been installed / activated for the first time or at subsequent updates

usable:
only on first install/activation

example:

define('WPPIZZA_NO_DEFAULT_ITEMS',true);/*bool | will not install any default menu items or categories*/
define('WPPIZZA_NO_DEFAULT_PAGES',true);/*bool | will not install any default pages other than orderpage and order history page*/
define('WPPIZZA_NO_DEFAULTS',true);/*bool | short for setting both, WPPIZZA_NO_DEFAULT_ITEMS and WPPIZZA_NO_DEFAULT_PAGES*/

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.