Message : Required fields (Reuters) - The impact of Apple Inc's new privacy controls over digital advertising tracking is likely to depend greatly on how app developers implement a new pop-up notification, Apple's user privacy chief told Reuters. Apple on Monday started enforcing a rule introduced last year requiring developers to use a pop-up notification seeking permission to gather data that can be used to track users across third-party sites and apps. Facebook Inc has said the rules could harm its customers. Some mobile advertising analysts believe fewer than one in three users will opt-in, which could diminish the effectiveness and profitability of ad targeting.