Debt investment occurs when one company lends another company money through bonds and notes payable. The lending company is a debt investor in the borrowing company.
Debt investments are classified as being either (1) held-to-maturity (HTM), (2) trading securities (TS), or (3) available-for-sale securities (AFS) based on the nature of the investment (i.e., what was investor's purpose in lending the money?).
The classification of the debt investment (into one of the three categories above), however, will change how we account for unrealized holding gains and losses as well as the value of the investment in the balance sheet.
The purpose of financial reporting is to provide information that is useful to financial statement users (e.g., investors and lenders). So what is deemed "useful information" in this situation depends on what the debt investor intends to do with the investment.