The default setting on an account is to save new content to your knowledge library. New content means you didn't use the search engine to copy responses out of the library. This could be content you authored or copied from an external source.
Note: Administrators can read about editing the settings here.
When you use existing content, choose to Save to Knowledge Base button turns itself OFF...so we don't put duplicates in your knowledge library.
If you modify a response you used from the knowledge library, you will have to turn the button on to save the new variation. The system doesn't make a decision based on an edit, as this could be one word, a sentence, or something very material.
If the response is approved, you can go back and click the Save to knowledge base button. That will push the question/response pair into the knowledge library.