As a user
When publishing a new content, a window will pop up to show you the list of available tags. If none of the available tags are relevant, you can create a new tag. There are two ways to do so:
Either via the "All tags" tab: Type in the search bar the desired tag, the system will propose you to create a new tag. You can then choose in which category of tags to store it or leave it in "Other tags" and it will then become an "orphan" tag, i. e. not belonging to any category.
Either via the tab of a tag category: Type in the search bar the desired tag, the system will propose you to create a new tag and this one will be placed directly in the right category.
When you create a tag, make sure beforehand that the idea you want to express does not already exist under another term; if you really think it is not in the list of existing tags ensure that you choose a term or expression which is:
short if possible
differentiating (which allows for filtering information)
explicit , unequivocal, unique
complementary to the search engine
as close as possible to the usual vocabulary of other users