What type of identifier is written in Block 9 D?

Block 9D on the CMS-1500 form is specifically designated for the Coordination of Benefits (COB) agreement information related to Medigap policies. This block is used to indicate the Medigap-based identifier that aligns with any existing coordination of benefits agreements. When a patient has more than one insurance policy, this identifier is significant for ensuring that the claims are processed appropriately between the primary and secondary payers.

In the context of the other options, while a Medigap policy number and insured's identification number are relevant pieces of information, they are recorded in different areas of the form. Similarly, a claims processing identifier would not typically be found in Block 9D, as it has a different purpose and location in the claims process. Thus, the focus on Medigap-related coordination makes this identifier crucial for the proper handling of claims when multiple insurances are involved.