A pool is a graphical representation of a collaboration participant. A pool serves as a container for the sequence flows that occur between activities (of a contained process). The sequence flows can cross the boundaries of a pool between lanes but not across the entire pool. In other words, a process is completely contained within the pool. Message flows depict the interaction between pools.^[1]

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1]     Silver, B. (2009). BPMN Method and Style: A levels-based methodology for BPM process modeling and improvement using BPMN 2.0. In CodyCassidy Press US.