A controlled vocabulary of keywords is one of the first aids in meta dating documents. As the absence of ambiguity is our objective, we focus on controlled vocabularies rather than free vocabularies (user tags, folksonomies et cetera).
This means, in plain English, that we lay the responsibility for the maintenance of this list of keywords with one or several individuals. If another person has a suggestion for improvement of the current list, this individual will check if the suggestion is valid and adjust the vocabulary accordingly. In organizations that thrive on meta data, a workflow process is usually implemented for communication between the team members.
From the moment a new keyword is added, other users will be able to meta date new documents with this added keyword. However, all previously meta dated documents will not receive this keyword until the next keyword revision.
In a list of keywords, each keyword has the same "weight"; there is no relationship between the keywords themselves.