We present a taxonomy of human-centered collaborative XR tasks. XR technologies have extended into the realm of collaboration, improving the quality and accessibility of teamwork. However, after a comprehensive assessment of the literature on the interaction between XR technologies and collaboration, no comprehensive method that emphasizes task actions and properties exists to classify collaborative tasks. Thus, our suggested taxonomy represents a classification system for collaborative tasks. After conducting a thorough literature review across different research venues, we conducted several exhaustive classification and review cycles for over 800 papers collected, which resulted in 148 papers retained to create the taxonomy. We dissected the actions and properties that the collaborative endeavors and tasks of these papers encompass as well as the types of categorizations and relations these papers illustrate. We expand on the design choices and usage of our taxonomy, followed by its limitations and future work. We built this taxonomy in order to reduce ambiguities and confusion regarding the design and comprehension of human-based collaborative tasks that use XR technology, which could prove useful in aiding the development and understanding of these tasks. Our taxonomy reveals a framework for understanding how collaborative tasks are designed and a systematic way of classifying different methods by which people can collaborate and interact in environments that involve XR, while still promoting efficient communication, teamwork, goal achievement and productivity.