Complex sentence: Scripts (and other blockable elements) are allowed or blocked based on the source from where the script is fetched-which very often is not identical with the URL displayed in the address field of the webpage ("main page").
Split into simpler sentences: Scripts (and other blockable elements) are allowed or blocked based on the source from where the script is fetched. Very often, this source is not identical to the URL displayed in the address field of the web page ("main page").