A "casing trim" describes the product category and doesn't really refer to any single product by itself, but instead covers both casings and trims, usually in the context of interior moldings that are placed around windows and doors.
Usually, a casing or trim is referring to the same thing and these terms can be used interchangeably. There is a slight difference in the sense that trim usually includes a design or aesthetic element, rather than just serving a practical purpose. In the case of this Casing Trims or Casing Molding product range, we include all of the products in the one place.