This is the Global Error Page URL to which the visitor is redirected to, if the content being visited is part of multiple products.

NOTE: This is used only if DAP cannot figure out, at a product level, as to which error page URL to choose when the page or post being accessed is part of multiple DAP products.

This is a global setting. You can also over-ride this at a Product level, by modifying the "Sales Page URL" on the Product page.