When you apply a modifier to a selection set, the same modifier is carried on the stack for each individual object. These are instanced modifiers: they are all exactly the same, and a change to the instance for any one object will change all the others.
In the stack, the name of an instanced modifier appears in italic.
Objects sharing a single instanced modifier
Identifying Instanced Modifiers
You can quickly lose track of which objects share the same modifier. An option on the Views menu highlights those objects.
To identify objects sharing instanced modifiers:
You can make changes to an entire set of objects from a single instance. This is a major advantage of instanced modifiers.
To adjust instanced modifiers:
The single object highlights and the appropriate gizmos appear for the entire selection set. Adjustments to this modifier now affect the entire set.
Changing the parameter of an instanced modifier for one object affects all the objects sharing the modifier.
Making Instanced Modifiers Unique
At some point in your work, you might want to turn a modifier instance into a local copy that affects only a single object. To do so, click Make Unique on the Modify panel. This button appears beneath the modifier stack display. (Make Unique is also available as a pop-up menu choice when you right-click the instanced modifier's name in the stack display.)
To make an instanced modifier unique:
The modifier is now separate from the set of instanced modifiers. Adjustments you make to this modifier no longer affect other objects. Its parameters and gizmo remain unchanged from their original, instanced settings until you adjust them.
To make multiple modifier instances unique:
There can be more than one instanced modifier in this stack. Click the one you want to make unique for each of the selected objects.
The parameters for this modifier disappear, because the objects no longer share the modifier. For each object, the modifier is now separate from the set of instanced modifiers.
As with a single object, the parameters and gizmo are unchanged in the now unique modifiers.