For example, that function can control specific keywords and related terms.
Then that groups function as standard or authority.
On the other hand, It helps public users can search items.
If it provides many keywords, users can choice a word what they want.
Then the word shows related terms and items.
In terms of creating a controlled vocabulary, you might look at the plugin Simple Vocab which allows you to create controlled vocabularies for specific properties. In this case, you would create the terms and users creating content can only use these terms.
There is also the Library of Congress Suggest plugin, which provides possible inputs but for properties, but users can write something else entirely if they want.