Below are 2 links with very good explanations, thanks to Jonathan Lewis!!!
https://jonathanlewis.wordpress.com/tablespace-hwm/
https://jonathanlewis.wordpress.com/2010/02/06/shrink-tablespace/
These help when you get:
ORA-03297: file contains used data beyond requested RESIZE value
but there is a lot of free space in the datafile, just not compacted.
