The strap attached along the bottom edge of each upper is how the uppers are convertible from the base. The fastening strap will go along the same colored strip around the top of the base, so it will not be noticed.
Front styles are usually accompanied by a back upper, but some can be worn alone like a slide or mule. They cover from the front region to the middle of the foot.
Back styles cover from the ankle point to the back region of the shoe styled with an ankle strap. Back styles are always accompanied by a front styles.
Full styles are stand alone uppers covering the majority of the shoe. These styles do not need an accompanying style like the single front and back styles.