According to your readings how is 'underuse' 'overuse' and medically necessary defined? Why is this important to know?

Providers and payers use UM to eliminate underuse and overuse of medically necessary healthcare services. The purpose of UM is to ensure that patients receive only medically necessary services at the least possible cost. Clinical practice guidelines, research evidence, and professional consensus are considered when identifying overuse and underuse of healthcare services.  According to your readings how is 'underuse' 'overuse' and medically necessary defined?  Why is this important to know?