If you dragged or imported the file, the file itself becomes a defintion (+1)
If you create a component, you ‘define’ its content by what’s currently in the selection, hence definition.
If you have checked ‘replace selection’ you will also ‘create’ an instance of that definition. Naming the instance in the entity info doesn’t alter the definition, you can have four instances of a (table) leg a call them Left-front, right-front etc) without making them unique.
It is hard to tell what exactly is going on in your files without actually seeing them…