In file: /magnolia-module-form/src/main/resources/info/magnolia/module/form/messages_en.properties

are the following keys, but there is no reference elsewhere of this elusive "link" field for a form type.
It is not used in demoproject/demofeatures, it is not refefrenced in documentation, there is no ftl file for it. So they should be removed from the language files as they cause confusion.

dialog.form.link.tabMain.controlName.description = If empty you won't be able to reference the field as used in the form
dialog.form.link.tabMain.controlName.label = Form Name
dialog.form.link.tabMain.link.description =
dialog.form.link.tabMain.link.label = Link
dialog.form.link.tabMain.linkType.description =
dialog.form.link.tabMain.linkType.label =
dialog.form.link.tabMain.linkType.option.internal = Internal link
dialog.form.link.tabMain.linkType.option.external = External link
dialog.form.link.tabMain.text.description = Empty text: External uses link, internal uses title of destination page
dialog.form.link.tabMain.text.label = Link text
dialog.form.link.tabMain.label = Link