We have an object A that is physically attached to object B. What can we call object A? I would have expected to be able to use "attachee", but this is not in the dictionary, or "attaché", but this seems to only apply to a member of an embassy. What can we call object B? Is there a word for "attached to"?
The question is Words to describe attached items
The answer to this is an attachment However, I am not sure if this is the word you are looking for.
Accessory or Addition could also be used in this context.
Attachment noun (PART) an extra piece of equipment that can be added to a machine: C.E.D