There are 3 "listening modes" for Prizm: discover mode, library mode and a mix of both.
When you are in discovery mode, Prizm will make you discover new songs. When you are in library mode, Prizm just plays music from your personal library.
And Prizm learns when you want to discover new songs and when you want to hear your own music.
For example, when you come back from work, you can set the library mode thanks to the Prizm app (cf Prizm app screenshot at the bottom right). Thus, next time you will be in the same context (coming back from work), Prizm will automatically set the library mode. Prizm learns from all your interactions!