'Special elements' contain elements for adding HTML-code, displaying a sitemap, adding a divider in between elements and creating referenced content on the site. (5.5.1)
Sometimes you need to embed information from other places, like a mail subscription form or a video clip from Youtube. In these cases you need to use the "Plain HTML" element to add the information. The retrieved code is pasted into the element.
The "Divider" element adds a thin line in between two content elements. When creating this element, you only need to choose the icon (if you need it) and just 'save and close'. No specific settings are needed.
By using the "Insert records" element, you can mirror content to other pages by creating a reference. For example, if you have an information box that you wish to have on multiple subpages, but only want to have to edit it in one place. Then you can create a reference, instead of a copy, and by editing the original element, it will automatically be updated in all the references as well.
When pasting a copy on a page, TYPO3 will ask you if you wish to paste a copy or a reference.