The Taxonomy autotagger will automatically tag a node with a term, if that term is present within the text of the node. In order to enable autotagging of nodes with a classification, you need to ensure that your taxonomy satisfies the following criteria.

• The vocabulary was created by the taxonomy module, and not by an installed module.
• The vocabulary is NOT a "Tags" vocabulary.requirement removed
• The vocabulary has "Multiple select" enabled.
• The vocabulary is NOT marked as "Required".
• The vocabulary is associated with the node content type that you're editing.
• Autotagging has not been disabled for this vocabulary.

Features

By default this module provides a modification to the node edit form, displaying a large pane where you can choose which autodetected tags to apply to a page. This edit form is placed on a second stage of the node edit form, Saving a node now takes two steps. (and may conflict with advanced node forms)

You can disable this behavior in the settings /admin/settings/autotag and just trust autotag to do the right thing whenever you save a node.

Autotag provides a new 'action' that becomes available in the content management screen, where you can apply autotag to many pages at once.
This action is also available to views bulk operations so you can retroactively tag everything in your site. #483174: How to re-tag all content or you can use the built-in "Re-Autotag by content type" tool on the settings page.