There is a system called the Monster Book in the online game

Monster Book: In Korea-MS, Japan-MS, China-MS, Taiwan-MS, Thailand-MS, and Global-MS, there is a system called the Monster Book. Monster Book is, as the name suggests, a book which buy ro zeny every player is given that catalogs monsters.

When the Monster Book is added to the game, monsters will begin dropping "cards" that contain detailed information on them. Each individual monster's card can be collected up to five times.

Upon picking up a monster's first card, the monster's slot in the book lights up and it is standing sprite becomes viewable.

The second time a card is picked up, the monster's HP and MP values will be displayed, as well as some new animations of the monster, such as walking or knockback.

The third time, the monster's history and level will be added. For the fourth card, a near complete drop list is revealed, with certain items (such as Skill books) omitted.

The fifth card will reveal the monster's exact locations across the world, and grant a temporary 15% experience boost. This boost lasts for one hour, starting from the moment the card is picked up.

All remaining animations for that monster will also be unlocked. Though cards will continue to drop after the final card is found, picking up more than five will have no effect.