ios App creating a local file on mac

Helle guys,

Form this page , we know that each app is an island, which means that an app is generally prohibited from accessing or creating files outside its containers. but also the page says One exception to this rule occurs when an app uses public system interfaces to access things such as the user’s contacts or music. In those cases, the system frameworks handle any file-related operations needed to read from or modify the appropriate data stores. Can any one presents an example of using public system interfaces in codes or point me to a page has such example? Thank you.