Unchecking the 'Controlled taxonomy?' option allows terms to be applied to the content and added to the taxonomy during page editing (in the Setup tab > 'Taxonomies' section).
In the screenshot below the 'Tell me about' taxonomy (1) is an uncontrolled taxonomy. Any user with the ability to edit this page (which could include Site Editors and Site Authors) can either:
apply the terms already listed in the taxonomy to the content;
specify a new term for the taxonomy, which will be applied to the content and added to the terms list when the settings are saved.
By comparison, the 'User role' taxonomy (2) is a contolled taxonomy. Terms listed for the taxonomy can be applied to the content but new terms can only be added on the 'List the terms for "[taxonomy name]"' screen (see section (3) 'Current custom taxonomies').
Page edit mode: Setup > Controlled and uncontrolled taxonomies
Choose which content type the taxonomy applies to (3)
Some taxonomies might only be relevant to content of one, two, or three types. You can specify which content types the new taxonomy should apply to in the Create new custom taxonomy screen. These settings can be edited later on the Taxonomy administration screen.
The Authors Taxonomy ((2) on the Taxonomy administration screenshot) is the only in-built taxonomy on a Mosaic site. The terms for this taxonomy are the names of authors of Article content on the site.
The Authors Taxonomy only applies to Article content. Terms are automatically added to the taxonomy when content is created using the Article content type and an author name is added to the article (in the article's edit mode: Details tab > 'Taxonomies' > 'Author' input field).
Taxonomy administration screen: Current custom taxonomy
View and edit the terms for a taxonomy (1)
You can view the list of terms for a taxonomy by clicking the 'List the terms of "[taxonomy name]"" link on the Taxonomy administration screen.
On the list of terms screen, the order of terms can be reorganised using the multidirectional arrows to the left of the terms. New terms can be added using the 'Add term' button.
List of taxonomy terms page
By clicking on the 'Add term' button, the screen pictured below will load. The term name (1) is mandatory; a description of the term is optional.
Add new taxonomy term screen
Once the new term has been saved, the add term screen (above) will reload to allow another term to be added. To navigate back to the list of terms, click on the 'List' tab at the top of the add term screen.