About DHCPSNP option82 format, what's the meaning of "Unit ID" ? Does it represent the order of switch connection ?
No, all the Edgecore switches unit ID start from 1.
For some stackable switches (ex, ECS4510, ECS4620), which may have 4 units in a stack for management. Then the unit ID is from 1 to 4.
If the client connects on port2 of second unit in stack, the interface would be "eth 2/2".