The singular form of "criteria" is "criterion." So, "criterion is" would be proper, as would "criteria are." "Criteria is" or "criterion are" would not.
The difference between desirable and essential is...essential is a MUST have where as...desirable is something someone would LIKE you to have.For example;in a job description it may have A-C in GSCE...
The last change to the Army of Occupation Medal (Germany) was in 1990 when the official occupation of Berlin ended (02 October 1990)
http://www.tioh.hqda.pentagon.mil/Awards/WWII%20OCCUPATION1.html