Although natively supported with Menubar > Object > Create Trim Marks, this script behaves differently:

• With paths and compound paths, the dimension of stroke is ignored.
• With clipped items, the clipping path dimension is used.

Single Target

Treats selection as one dimension and create trim marks around it.

Multiple Target

Taking account the dimension of all selected items (grouped items count as 1) and create multiple trim marks around outer bounds.

This feature is particularly useful after Step and Repeat.