Share Topology (Share Topology (ansys.com)) is used when bodies are grouped into multi-body parts and the solid(s) and surface(s) are touching or intersecting.If the bodies are Shared then the nodes will also be shared and thus the mesh.
When "Group" (Groups (ansys.com)) is used the multi parts are saved as individual parts and no node is shared between them. Thus Contacts will come up during analysis.
Both the approaches are suited for different conditions and the correctness of your analysis depends on your problem statement.
Please refer to this to get a clear idea - Contact or Shared Topology ÔÇö Ansys Learning Forum