A reader app is an app without in-app purchases or any other sign-up method. It is used as a way to access & view previously purchased content or content subscriptions including audio, music, and video. (see Apple's guideline definition here). They are primarily used as a companion to a core web offering to provide greater accessibility and UI on mobile devices. Since they don't have a way for users to sign up, they are not best for growing a user base.
How does this affect the functionality of my app?
To comply with Apple's guidelines on reader apps, the app will be locked by a sign-in screen so only users with an existing account can log in and view content. Any other users who download the app will not be able to preview any content nor access any links to sign up. You cannot provide any external links/URLs that take users to your website for sign-up as this violates the guidelines and Apple will reject the app. This is something Apple take particularly seriously
External Link Entitlement
Apple does, however, provide an entitlement for some apps to allow an external link to a web sign-up. You can find more info about this here. From our experience, Apple is very strict in giving these out so it's best not to expect to receive this entitlement.
Our recommendation
we recommend using Apple & Google's in-app purchases to provide seamless sign-up to users within your app. With in-app purchases, users will be presented with a purchase screen when first opening the app, where they can then choose to sign up, browse the content, or sign in to an existing account.