Depends on the train. Regardless of the train type you must always log the train on Rail Planner before boarding. This is your ticket. In addition:

Train with mandatory reservations: you must get a seat reservation or you'll be asked to leave + pay a fine ("travelling without valid ticket"). Onboard show both Rail Planner's QR code and the reservation PDF.

Optional reservations: board the train and look for untaken seats. There are usually paper slips or little screens above the seats. Show Rail Planner's QR code whenever asked.

No reservations: hop on, sit wherever you want. The QR code from Rail Planner is enough.