Share topology is used to get conformal mesh between different volumes.
When you use share topology between two volumes, you will get single face between two volumes. This face is shared by two volumes. So this common face will have cells on both sides.
If you do not use share topology between two volumes then you will get two faces at the connection. Each volume will have one face. Now as volume ends here, it will have cells only on one side. Meshing identifies the faces which are at the same location and creates contact regions. These contact regions can be used for defining interfaces and use it for non conformal mesh.
Please check the documentation for more info about share topo, conformal and non conformal mesh.
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