I was reading the player's book and in many parts of the book it says
The test is determined by the difficulty of the class
I read it several times and can't understand if the difficulty class is defined by the character's background, or if the DM defines this test and its value.
This was taken from an excerpt from the book
You know Druidic, the secret language of druids. You can speak the language and use it to leave hidden messages. You and others who know this language automatically spot such a message. Others spot the message's presence with a successful DC 15 Wisdom (Perception) check but can't decipher it without magic.
The question is, what is this difficulty class and how is it determined?