What is the "application identifier" ?

The application or bundle identifier is used in the actual build process of an app, and it is not required for a "Preview" (or "simulator") build.

The recommended practice for an application identifier is to use a reverse-domain name style string.

An example App identifier would be: com.twixlmedia.twixlmagazine or in general com.[your_domain].[your_product_or_publication].[issue number].

For more detailed technical information about application identifiers, please refer to Apple's documentation on the iOS Provisioning Portal.