Hi Rekha,
As the name suggests, JCos are Java Connectors. JCo RFC provider are used to connect from ABAP to Java system.
Basically ABAP R/3 system connects to PI SLD using JCO RFC to access the Business system details.
While in SAP PI dual stack systems we used to have normal RFC destinations as R/3 of ABAP used to connect ABAP stack of PI systems.
Regards,
Nitin