This MIB's description text for this object appears to be a copy-paste artifact duplicated from the neighboring clock-backup object, but the object's name and enumerated values indicate it selects which standard, ANSI or ATM Forum, and which cell class, invalid or unassigned, this port uses to classify and discard non-data ATM cells. Getting this classification wrong on this specific port can cause it to either discard cells that a strict ATM Forum-compliant far end considers valid unassigned cells, or fail to discard genuinely invalid cells, either of which shows up as unexplained cell loss or unexpected idle-cell leakage on this link. A port connected to ATM Forum-compliant equipment would typically be set to one of the atmF* values here so its discard behavior for unassigned cells matches what the far end expects, rather than following the stricter ANSI convention.