Bushing in plastic or rubber is an insulating bushing. Bushing in metal such as flange nipple is not insulating. In the first menu you will find the insulating bushings. In its submenu you will find flange nipples, threaded flanges, in both insulating plastic and non-insulating metal bushings.
Strain relief is, as the name implies, a relief for the lamp cord. Here too, a strain relief can be a splice sleeve in metal and they are also found here because they are insulated. The common denominator for strain relief is that they are protective-insulated (except that they are strain relief for the lamp cord.)