This object identifies what kind of interface, such as a physical port, LAG, or virtual circuit, a given MEP is bound to, and per its description it also serves as part of the table index that uniquely identifies each MEP row alongside the interface index below it. Because a MEP's behavior and reported counters are only meaningful in the context of which interface type it is attached to, this value is what an admin uses to correctly interpret a MEP's stats; a MEP on a LAG-type interface, for instance, is more likely to show up-order or out-of-order loopback replies due to hashing across member links. Seeing this field read 'aggregate' rather than 'physical' for a given MEP row immediately tells the admin that MEP's traffic could be spread across multiple physical links.