Few bugs can afftect the application highly but put on hold 
to fix later. Take an example: suppose you are filling a 
registeration form and submitted successfully but after 
entering 2000 details it started showing unexpected errors 
like server error etc. Now, during the time of 
categorisation under priority we must understand that this 
is affecting the functionality but after submitting 2000 
details means 2000 details been submitted successfully. 
Therefore this issue will consider under high severity and 
low priority.

For a finance application the login and password screen is 
not developed initially. During internal testing of the 
application the development is not required. But must be 
created and tested when the application moves to the real 
world. This is low priority as this development can be done 
before the delivery to the client. High severity, because 
it is a finance matter.