PRN (Permanent Registration Number) is a 16 digit unique number that is also referred to as the roll number of a student provided by this university. 

While applying for any course at Mumbai University, you are required to fill an online application form and after submitting this form the data is stored by the system. The data is used to find whether or not you are eligible. If you are eligible for admission then you will be provided with the 16 digit number known as PRN and it will remain the same throughout the registration period.

This PRN number can be used as a reference number for any kind of contact or communication with the university

So, the PRN number of a student is basically the reference or roll number which will be used throughout your graduation period.