Block change tracking

when we enable block change tracking, by default an OMF file will be created in DB_CREATE_FILE_DEST location with the initial size of 10M and will grow in 10M of size. This file will record the information of the blocks which are getting changed including their block SCN. We can use already existing file also as change tracker file as follows:
ALTER DATABASE ENABLE BLOCK CHANGE TRACKING USING FILE ‘/u01/oradata/MYSID/rman_change_track.f’ REUSE;