Per my chat with client care, in the debugger, under Launch configuration, "Add Embed Code" is intended for use when there's no script present, whereas "replace" or "block" is hidden under the "...". We use these features to debug our sandbox environment on live site.
I understand the difference between the two, but could we make it clearer or give replace its own button and add tooltips?
The obvious thing to do is to click the big button right in front of you, when in reality you need to block/replace the active script to accomplish what you want
"The important distinction is that the inject option is intended for pages with no embed code, whereas the replace option should be used if there is already an imbed tag present."