These three fittings solve different jobs, so check your existing bulb before you buy. G9 is a small push-fit capsule bulb with two pins close together, commonly used in compact ceiling fixtures, wall lights, and some table lamps. GU10 is a twist-and-lock bulb with two pins and a wider base, almost always found in directional spotlights and recessed downlights such as bathroom and kitchen ceiling spots. Light Engine is a technical light source designed to sit inside a decorative diffuser or shade rather than be seen directly, so if your fixture has an integrated glass or fabric diffuser rather than a visible bulb, a Light Engine is likely what you need.