The subperitoneal space and peritoneal cavity are two mutually exclusive spaces that are separated by the peritoneum. Each is a single continuous space with interconnected regions. Disease can spread either within the subperitoneal space or within the peritoneal cavity to distant sites in the abdomen and pelvis via these interconnecting pathways. Disease can also cross the peritoneum to spread from the subperitoneal space to the peritoneal cavity or vice versa.
Pannu, H. K., & Oliphant, M. (2015, October 29). The subperitoneal space and peritoneal cavity: basic concepts. Abdominal Imaging. Springer New York LLC. https://doi.org/10.1007/s00261-015-0429-5