I can reserve a placed object in the project, and I can also reserve an Embedded Object from the Library Manager (or Object Editor). What’s the difference?
These two reservations are entirely unrelated. When you reserve a placed instance of an object in the project, you can then edit its object settings, either in place or in the settings dialog box. Your edits will affect only the particular object instance you have reserved. It is possible to reserve an Embedded Object itself (as opposed to its placed instance), either in the Library Manager, or in the Object Editor. (Both locations will indicate the object’s reservation status; they are in sync.) You can then edit its script in the Object Editor. Editing the script will affect all instances of this object in the project, regardless of who has reserved those instances.