Implants are a new type of magical item, similar to symbiotes but crafted as a magic item instead of grown as an aberration. What makes implants different than other magical items is that they must be implanted onto the recipient through surgery and this surgery has a mandatory requirement that the recipient loses the equivalent body part(s) and replaces it with the implant.

Whether or not the recipient is already missing the necessary body part or if it is removed during the process of implanting the implant is unimportant. Both are equally valid options for getting an implant. Once attached, the implant is considered to be part of the recipient.

Rules page on Eberron Pathfinder.

While daelkyr experiments with symbionts proved that magical modification of a body is not new, factions across Khorvaire began a new type of magical experimentation and research a few years before the end of the last war, with the new magic having yet to spread beyond a few select nations. Implants, created by taking a portion of an existing creature and transplanting it into or onto the body of another, were viewed as a safer way to grant an individual greater inherent magical power, while having a peripheral similarity to symbionts.

Implantation is a complex and, for the most part, new method of item creation, and its many benefits are still being fully explored, requiring a different type of expertise for each of its types. Implant bearers can gain the powers of many magical item types without actually wearing those items, and due to the fact that many are difficult to spot through casual observation, they allow greater stealth opportunities.