What constitutes medical necessity?

What constitutes medical necessity?
Documentation in the patient record serves as the basis for coding. The information in the record must support codes submitted on claims for third-party payer reimbursement processing. The patient’s diagnosis must also justify diagnostic and therapeutic procedures or services provided. This is called medical necessity and requires providers to document services or supplies that are proper and needed for the diagnosis or treatment of a medical condition.
The diagnosis pointer letter: item letters A through L preprinted in Block 21 of the CMS-1500 claim; the letter next an entered ICD-10-CM code in Block 21 is entered in Block 24E to indicate medical necessity of a procedure or service performed.
It is the responsibility of the medical biller and coder to establish medical necessity. As a biller and coder, correct claim submission is a must for timely payer reimbursement.
For your initial post, please provide explanation how box 21 and 24E established medical necessity. A minimum of 250 words with reference citation.