setFieldHidden() is used to specify the value of the VISIBLE field attribute of the field or fields with the given name. It has basically the same effect as the Visible property of a theme file:
MAIN
DEFINE wo ui.Window
DEFINE fo ui.Form
DEFINE prod_id INTEGER
DEFINE article CHAR(20)
OPEN WINDOW orders WITH FORM "ord_list"
LET wo = ui.Window.getCurrent()
LET fo = wo.getForm()
INPUT BY NAME prod_id, article
ON ACTION ("hide")
CALL fo.setElementHidden("idlabel",1)
CALL fo.setFieldHidden("products.prod_id",1)
ON ACTION ("show")
CALL fo.setElementHidden("idlabel",0)
CALL fo.setFieldHidden("products.prod_id",0)
END INPUT
END MAIN
It accepts two parameters: first is the name of the field and the second is the value of the VISIBLE attribute. The value of the VISIBLE attribute can be either 0 (then the field will be visible) or 1 (then the field will be hidden).
ui.Form.SetFieldHidden() has a synonym - ensureElementHidden() - used for compatibility purposes.