What if I change a price or a quantity on my shop between two files?

The review of the next file shows it: for every price, sale, sale date and stock quantity the file sets, it reads the shop, and a product whose value there differs from the file is listed as On the site now against This file. By default publishing then puts the file's value back, because the shop is meant to end up as the file says.

You choose that for each field. Under Update it when, the site's value differs from the file is the default and fits prices. The file changed since the last file compares only with the last file, so a number someone changed on the shop stays until the file's own number changes: use it for Stock quantity when the shop sells products between two files. Prices, stock and sales explains both, and the weekly price tutorial shows them in the review.