For the Footballer’s Journey collection each digital collectible of any rarity is called a Panel. Each pack has 4 panels.
To complete a Chapter you need to have its 7 Panels. The whole chapter is also a digital collectible. In terms of rarity, each chapter has 1 rare panel.
To complete a Book, you need to complete its 4 Chapters. In total there are 28 panels per Book. Each Book is dedicated to a single player.
You can find all details in the collection’s Litepaper.