nt:hasPrefix puts a fixed beginning in front of a URI field, so the user only types what comes after it, and nt:hasPrefixLabel gives that beginning a readable name in the form.

The prefix may also contain a token standing for wherever the form was opened: ~~SPACE~~ for the current space, and ~~NAMESPACE~~ for the namespace of the current maintained resource. A template using these mints new resources under the space or vocabulary the user is working in, rather than under a namespace fixed when the template was written. If the form is opened without such a context, it shows a picker to choose one.