Select an editable mesh object. > Modify panel > Attach button
The Attach Options dialog appears when you attach two or more objects to which materials have been assigned. It provides three different methods of combining the sub-materials and the material IDs in the two objects.
Interface
Match Material IDs to Material: The number of material IDs in the attached objects are modified so they are no greater than the number of sub-materials assigned to those objects. For example, if you have a box with only two sub-materials assigned to it, and you attach it to another object, the box will have only two material IDs, instead of the six it was assigned on creation.
Match Material to Material IDs: Maintains the original ID assignment in the attached objects by adjusting the number of sub-materials in the resulting multi/sub-object material. For example, if you attach two boxes, both assigned single materials, but with their default assignment of 6 material IDs, the result would be a multi/sub-object material with 12 sub-materials (six containing instances of one box's material, and six containing instances of the other box's material). Use this option when it's important to maintain the original material ID assignments in your geometry.
Note: If you want to make the instanced sub-materials unique, select them in Track View, and click the Make Unique button on the Track View toolbar.
Do Not Modify Mat IDs or Material: Does not adjust the number of sub-materials in the resulting sub-object material. Note that, if the number of material IDs in an object is greater than the number of sub-materials in its multi/sub-object material, then the resulting face-material assignment might be different after the attach.
Condense Material IDs: Affects the Match Material IDs To Material option. When this is turned on, duplicate sub-materials or sub-materials that aren't used in the objects are removed from the multi/sub-object material that results from the attach operation.
Tips
In most cases, use the first option (Match Material IDs to Material) while keeping Condense Material IDs selected. This maintains the appearance of the objects, and results in the fewest additional sub-materials or IDs.
Use the second option (Match Material to Material IDs) when you need to maintain the original material ID assignments.
Leave Condense Material IDs selected unless you have an unassigned sub-material that you want to keep for future assignment.
Perform Edit menu > Hold before performing the attach.