I hope you are referring to BEM.
Default is BEM cell itself. It receives all the events unless and until you create any other cell as per the requirement/for receiving the events of any specific integration.
SIM cell is just another cell created manually for receiving only SIM-specific events.The command is same for creating these cells i.e mcrtcell command and after that use iadmin command for registering the cell.Refer to Page 204 of the attached guide.
In older versions, there was actually a different default kb directory between a BEM and SIM cell. Now, since you can publish/create CI's to a BEM cell for Component based policies, it uses one standard kb for both, and the difference is in the ServiceModelEnabled parameter in the mcell.conf. With ServiceModelEnabled set, the cell can perform full SIM functionality with the model (such as propagating status through relationships between the CI's).
The difference in the mcrtcell syntax is either -ae (BEM) or -as (SIM). The default with mcrtcell is a BEM cell, see page 266 at http://documents.bmc.com/supportu/documents/77/17/97717/97717.pdf.