One count between the till and the shop
WooCommerce holds the website, Square holds the room. Neither reads the other.
Square keeps its own catalogue for the register and WooCommerce keeps its own for the site, and neither has any idea what the other has sold. Left alone they separate quickly, and the moment you notice is usually the wrong one: someone buys online the last of something that just went through the till.
QuickSync holds one count between them and corrects each side from the other, so neither platform has to be the master. A counter sale lowers the website; a web order lowers what the register will let you sell. Both sides announce their changes to us immediately, so neither is waiting on a scheduled check.
Items pair up by SKU and Square is the tighter of the two at 80 characters against WooCommerce’s 300. That is roomy enough that most catalogues never hit it, but it is worth a glance at your longest SKUs before you connect, because anything over the limit gets truncated on the way to Square and then stops matching.