Modifier 51 applies when:

Modifier 51 is used to indicate that multiple procedures were performed during the same session by the same provider. This modifier is applicable when the provider performs multiple surgical procedures that are not staged or bundled together. It helps in reporting and reimbursement by signaling that the listed codes involve separate procedures that should be acknowledged for payment, albeit recognizing that the additional procedures may be subject to a reduction in payment (as not all services are paid at the full rate due to the lesser complexity involved in subsequent procedures).

The use of modifier 51 is particularly relevant in surgical settings, where multiple approaches to treatment might occur. Therefore, by appending modifier 51 to the secondary procedures, it communicates clearly to payers that more than one distinct procedure was conducted.

The other options do not accurately capture the purpose and application of modifier 51. For instance, bundling of codes is addressed under different modifier guidelines, and modifier 51 specifically does not apply in scenarios where only a single procedure is performed or when no procedures are performed at all.