In general, metadata source plugins will take the first of the following that retrieves a book:
- Use the existing identifier for the site.
- Use some other identifier if the site supports it (Goodreads can use ASIN).
- Use the existing ISBN, if the site supports ISBN search.
- Using the title and author.
- Use just the title.
Exactly which of the above is used, depends on the metadata source plugin. For Goodreads, all except the last is done.
The plugins may check the title and author against the results. It depends on how the results are presented by the site.