Sign up categories are used to group your different workplaces into categories that users can apply for.
An entry category consists of:

For a category you can associate workplaces - this is done at the individual workplace.

You can find your sign up categories under: "Settings" - "Edit sign up" - "Registration categories". Here you can add new and edit existing ones.
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PS: You can first add tags to an category when the category has been created.