This sub-rule under In-App Purchase governs external links — buttons, banners, or text that direct users to purchase digital goods or services outside the app aren't allowed unless the app qualifies for one of Apple's specific exceptions, like Reader apps or certain regional entitlement programs.
A common trigger is a "manage your subscription on our website" link that also doubles as a cheaper alternative checkout, rather than pure account management.
"Your app includes a link to a purchase method other than in-app purchase for digital content, without qualifying for an applicable entitlement, per guideline 3.1.1(a)."