Round caps are smooth and neat to match most any standard pipe/insulation dimensions.
Half caps are designed with extra material for slitting the cap and resealing without the need for “tabs”, scribed with various centerlines to ease fabrication for simple reducers or pass through pipes.
Reducing Caps step down from one outside diameter to another. They can be used on most PIC elbows, tees, and victaulic fittings. Price for reducing caps are determined by the larger outside diameter.