Mike, a new medical assistant, is working with the ICD-10-CM manual and is trying to refresh his memory about nonessential and essential modifiers. In your own words, define essential modifier and nonessential modifier. Mike is reviewing the Tabular List of the ICD-10-CM coding manual. He notices that each chapter begins with a list of diagnostic categories with a corresponding range of codes. What is this called? How can this feature be used as a tool for accurate ICD-10-CM code assignment?