legacy data generally required

hi friends,

1) I want to know that what all different datas that are generally required from the legacy system to be migrated to SAP ISU.

for Eg. business partner , contract account , contract etc....

I mean to say basically cut over data during implementation

2) which fields are generally kept mandatory.

3) how it uploaded in SAP ISU. (for e.g. BDC , LSMW, EMIGALL etc)

4) is the data for business process like move in , open billing docs etc is also uploaded & how?

Your inputs are valuable ...

Regards,

Chetan