In the industrial Internet of Things (IIoT), the concept of digital platforms has received significant attention. Although IIoT platforms revolve around similar business objectives, they address a variety of use cases and, thus, differ considerably in their architectural setup. While research has already investigated IIoT platforms from a business or design perspective, little is known about their underlying technology stack and its implications. To unveil different IIoT platform configurations and better understand their architectural design, we systematically develop and validate a taxonomy of IIoT platforms' architectural features based on related literature, real-world cases, and expert interviews. On this foundation, we identify and discuss four IIoT platform archetypes. Our findings contribute to the descriptive knowledge in this ambiguous research field, while also elucidating the interplay of IIoT platforms' architectural setup and their purpose. From a managerial viewpoint, our results may guide practitioners in comparing and selecting a suitable IIoT platform.