Hi THenri,
I hope you've have found your answer cause I'm having the same question as yours now. I assume there are two possibilities about priority deadline. One is that the applicants applying before the priority deadline can expect an early response from the university informing the admissions decision. The other is that, just like how early decision works in some US undergraduate schools, if the applicants applying before the priority deadline got the offer, then the applicants would have to go to that program. In the second case, the applicants don't have the right to decline the priority deadline admissions and would have to turn off all the other offers from other programs.
But I'm not sure if any of my two assumptions is accurate. So please let me know if you've got the answer to this question.
Thanks.