The person specification will state the minimum essential skills, knowledge, personal qualities, education and experience you will need to have for effective performance in the job, and will be used as the basis for selection for interview.
Desirable Criteria stipulated on the person specification are those qualities, which would be an 'added bonus' and help you carry out the full range of duties more readily. Where there are too many applicants for a reasonable short-list, regard may be given to the desirable criteria.
The selection criteria in all cases will be consistent with the complexity and level of the post.
If there are a large number of applicants we may not be able to shortlist everyone who meets both the essential and desirable criteria therefore the quality of your application will be important.