You can use 'Group Marker' FLEXIcontent field, to create tab-sets or field-sets in your item edit forms.
In order to achieve this, the group marker fields must be placed carefully using the following procedure:
1. Filter by a content type e.g. "car" or "article"
2. Order the fields by click at the column header of the Ordering column
3a. For Field-sets, create groumarker fields and order like this:
Fieldset: Open (with title1)
-- some fields --
Fieldset: Close
Fieldset: Open (with title2)
-- some fields --
Fieldset: Close
Fieldset: Open (with title3)
-- some fields --
Fieldset: Close
3a. For Tab-sets, create groumarker fields and order like this:
Tabset: Start
Tab: Open (with title1)
-- some fields --
Tab: Close
Tab: Open (with title2)
-- some fields --
Tab: Close
Tab: Open (with title3)
-- some fields --
Fieldset: Close
Tabset: End
4. Open item edit form to view the result and correct any bad formating by going back to field manager
NOTE 1: Be careful with placing group marker fields as you can easily mess-up the formating of your item edit form
NOTE 2: Avoid placing required fields inside tabs, because currently the tab will not be auto-focused, if user clicks submit while not having filled in the required fields inside the tab
NOTE 3: All core fields have specific place and cannot be placed (via group markers) into tab-sets/field-sets/separators. The exception is description CORE field which is displayed with the custom fields, so it can be used with tab-sets/field-sets/separators