8 classes are annotated with FieldGroupFormatter in Field Group 8.3

Accordion in src/Plugin/field_group/FieldGroupFormatter/Accordion.php

Plugin implementation of the 'accordion' formatter.

AccordionItem in src/Plugin/field_group/FieldGroupFormatter/AccordionItem.php

Plugin implementation of the 'accordion_item' formatter.

Details in src/Plugin/field_group/FieldGroupFormatter/Details.php

Details element.

DetailsSidebar in src/Plugin/field_group/FieldGroupFormatter/DetailsSidebar.php

Details Sidebar element.

Fieldset in src/Plugin/field_group/FieldGroupFormatter/Fieldset.php

Plugin implementation of the 'fieldset' formatter.

HtmlElement in src/Plugin/field_group/FieldGroupFormatter/HtmlElement.php

Plugin implementation of the 'html_element' formatter.

Tab in src/Plugin/field_group/FieldGroupFormatter/Tab.php

Plugin implementation of the 'tab' formatter.

Tabs in src/Plugin/field_group/FieldGroupFormatter/Tabs.php

Plugin implementation of the 'horizontal_tabs' formatter.