This has no description text in the source MIB, but its name and enumerated values, atmforum and itu, indicate it selects which standard, ATM Forum or ITU, this port follows for the format of the ATM idle cell it transmits. The idle cell header pattern differs slightly between the ATM Forum and ITU conventions, so mismatching this on this specific port against what connected equipment expects to see during idle periods could, on strict implementations, cause idle cells to be misinterpreted rather than simply ignored. A port interconnecting with ITU-aligned carrier equipment would be set to itu here rather than the ATM Forum default, to keep idle-cell formatting consistent with what that carrier's gear expects.