Hi Federico ,
Employee creation will happen in SAP ERP.
here are two thought processes for your concern -
1 ) If your customer does not need your SAP HCM system to be updated until after the Onboarding process , the trigger should be from RCM
2) For customers who want their employees early in the process trigger from SAP HCM. Mostly in the case of a future hire scenario.
There is more information available on - http://help.sap.com/saphelp_erp_sfi_addon30/helpdata/en/03/20d85472bad877e10000000a423f68/content.htm?frameset=/en/ba/53ee501b22cc26e10000000a441470/frameset.htm¤t_toc=/en/9b/39541ea628461da1119d1b66bf4011/plain.htm&node_id=39&show_children=false
Thanks & Regards,
Ankita