If the target environment already exists BCV5|6 performs a compatibility check. It checks the compatibility of all involved objects, but also recognizes that in the target may be additional indexes. In this BCV5|6 automatically generates a REBUILD index statement and performs that after the copy process is done. The REBUILD index is necessary because BCV5|6 replaces the existing data of the tablespace and DB2 is not involved in that process. So the data of the index is not valid anymore.