Which use case / situation need this config in MC-LAG setup?

Hi all,

Can someone explain to me in which use case/situation below knob "bold" need to use in MC-LAG setup? I try to google but not found any good explanation.

irb {
unit 300 {
description "ICCP Connection to DC2-MX-5";
family inet {
address 10.42.14.3/24;
}
mac 00:11:10:42:34:03;
}
unit 104 {
description "Connection Layer 3 to DC2-QFX-5";
family inet {
address 10.40.114.2/24 {
arp 10.40.114.3 l2-interface ae0.0 mac 00:00:42:13:40:03 publish;
arp 10.40.114.4 l2-interface ae1.0 mac 00:00:42:13:40:04 publish;
}
}
mac 00:00:42:13:10:02;
}
}

Thanks and appreciate any feedback