Below is the ROWID format fro Oracle7

'BBBBBBBB.RRRR.FFFF'

where
BBBBBBBB is the block number,
RRRR is the slot(row) number, and
FFFF is a file number.

So in yur query you are counting the no. of occupied blocks in all the files.

However the ROWID format has been changed in Oracle8

Sanjay