What is the difference between Predefined field and Boundary Condition ?

https://www.youtube.com/watch?v=VaDm20OJQJQ

 

What is the difference between Predefined field and Boundary (initial condition) condition ?

 In this example,

This model include predefined field(speed) in the solid bar and Boundary Condition(angular speed) at wheel hub.

This model works when I replace predefined field  with boundary in solid bar.