Heat & bond/ wonderunder are fusibles used for appliqué, they are not interfacing. There is fusible interfacing, where one side is fused to the back of the fabric to give it stability. It comes in different weights and fibers, it stays in the project, is usually used in thin fabrics, silky, or stretchy to keep them stable & to hold their shape.
do you need interfacing? Or do you need a fusible web? Fusibles like wonder under, heat n bond light & steam a seam 2 wash away after your project is completed and laundered so the appliqué needs to be stitched, the fusible is just used to hold it in place until stitching is complete