How processes communicate with each other

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Hello everyone, I would like to know how MFEM handles inter process communication issues in parallel computing. Taking Maxwell in Miniapp as an example, it seems difficult for me to detect data exchange between processes. In computational electromagnetics, transmission conditions (FETI) or numerical flux (DG) are commonly used for domain decomposition, so it is very important for me to understand the theory on which inter process communication is based. I look forward to receiving answers or recommendations for relevant reference materials. I would like to express my sincere gratitude to you!