Both of these features will allow you to place standard text at the bottom of all jobs. But there are differences with how they work.
Settings>Editing>Default text for the Additional Information text area on all new jobs. Entering content into this field will pre-populate the Additional Information field for all NEW jobs posted. This field can be plain text, or you may enter HTML code to allow for text formatting and insertion of images or links.
Additional info is populated on the jobs when they're posted. If you remove the default Additional Info, it will no longer post on new jobs, but it will remain on existing jobs.