The backing of a custom patch determines how it will be attached and how it performs over time. Common options include iron-on, velcro, sew-on, adhesive, and magnetic backings.
Iron-on is best for permanent applications on cotton and polyester. Velcro allows patches to be swapped between garments. Sew-on provides the strongest hold for heavy-use items. Adhesive works for temporary applications, and magnetic backings protect delicate fabrics from needle damage.