Table 2.

Taxonomy of 8 additional aspects.

Item	Definition
Connection to social network	The app itself provides a connection to a social network (eg, Facebook, Twitter)
Advertisement	Pop-up or stationary advertisements are shown within the app
Registration or account	Registration is required in order to use the app or some functions of the app
Pure e-booka	The app consists only of text that may or may not be divided into different chapters
Test version	Payment or download of a full version is necessary to receive access to some features of the app
Internet connection necessary	The app only opens when an Internet connection is available
External links to other websites	Websites are linked in the app, or videos or audios only play with an Internet connection
Wearable	The app can also be used with a wearable device

^ae-book: electronic book.