This sets the specific data word used to fill the payload of ATM idle cells transmitted on this port when there is no real traffic to send. ATM links must keep transmitting a continuous cell stream even when idle, so this value on this port is what fills that gap; a nonstandard value here can occasionally cause interoperability issues if the far end validates idle-cell content. Most equipment defaults this to the ATM Forum-standard idle payload pattern (0x6A repeating), and changing it away from that on this port would only be done to match a specific far-end vendor's expectations.