Resource process - Append or Replace


What  are the criteria of Append and Replace while resource process? I mean how do I know whether I have to do Append or Replace in resource process. Is there a way to determine this? What are those rules for Append and Replace?

For example: Security groups have been migrated(mirrored) along with sidhistory from source domain to target domain during Inter-forest migration means source groups still exist in source domain. Domain Local groups in source domain(applied on resource ACL) have been changed to global groups in target domain. Apart from this, resource on source domain have also been migrated to target domain. User objects have been migrated along with sidhistory from source domain to target domain. Those source user objects have been deleted in source domain.

Please explain according to above mentioned example what should I choose Append or Replace?