Category is a metakey, so you are correctly using SetMetaData method of the IContent object. The CategoriesField control should be automatically populated with the value set for a category.
You can use the methods below to save a tag to a given content item. The difference here is that you can save multiple tags to a given content item
(Guid contentId, List<string> tags) - Saves tags for a particular content. The first parameter is the primary key of the content for which tags are being saved. The second parameter is List of tag names to be saved
(Guid contentId, List<string> tags, string owner) - Create/Updates Tags and assotiates them using TaggedContent, Optionally, owner is used to , know "My tags" list
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