The booking confirmation for your 'Toma de huellas' (fingerprinting) appointment at the Policía Nacional. The PDF includes the date, time, office address, your name, your NIE, and a booking reference code. You must bring it to the appointment — without it, they can turn you away.
Issued automatically at the end of the cita previa booking flow on https://icp.administracionelectronica.gob.es/icpplus/index.html. It also arrives by email if you provide one during the booking.