What is a coding convention used in ICD-10-CM that affects code selection?

The correct choice revolves around the phrase "code first," which is a coding convention used within the ICD-10-CM framework to guide accurate code selection. This instruction is essential because it provides direction on how to prioritize or sequence diagnoses appropriately. For instance, when a patient presents with multiple conditions, "code first" indicates that a specific underlying condition should be recorded before coding symptoms or secondary diagnoses. This ensures that the primary reason for the patient’s visit is adequately highlighted in the coding, which can affect the insurance claims process and the patient's medical record.

The use of coding conventions like "code first" promotes accuracy and consistency in coding practices, ultimately influencing data quality and analytical outcomes in healthcare settings. Other options, while relevant, do not directly impact code selection in the same structured manner. Explanatory letters primarily serve to clarify specific guidelines within the coding system, standard abbreviations assist in communication but do not define code selection processes, and defaults pertain to situations when a diagnosis is unspecified, rather than guiding how to select codes effectively.