In application for Cathay Pacific they say PRN not required for First applicant what do they mean?

How do you apply for Cathay you send their application form only or an application form with a resumee and cover letter?

Afther some thinking I came to the conclusion that PRN must be personnal registration number... That I imagine they will send me once they recive my appication... Am I right?

yup

If you send them a resume and cover letter you will receive an application form and PRN#... Or you will get your PRN# from them upon your first reply from them with an application form filled out...
Better have most of the morning to fill the thing in, and a couple years before you will start work if you make it through the recruitment process.... Good Luck