When an MLAG domain is created, you have to associate a pair of MLAG member ports on each MLAG peer device to form an MLAG link by using command set protocols mlag domain <domain-id> interface <lag-interface> link <link-id>.
For example, bind LAG port ae3 to MLAG link 2 of MLAG domain 3.
The paired MLAG member ports of the same MLAG must be bound to the same MLAG link ID. Different MLAGs are identified by different link IDs. For example we have two MLAGs in an MLAG Domain then link ID 1 could identify the first MLAG while link ID 2 could identify the second MLAG in the MLAG Domain.