PCTFREE: is a block storage parameter used to specify how much space should be left in a database block for future updates.

For example, for PCTFREE=10, Oracle will keep on adding new rows to a block until it is 90% full. This leaves 10% for future updates (row expansion). 

When using Oracle Advanced COMPRESSION, Oracle will trigger block compression when the PCTFREE is REACHED. This eliminates holes created by row deletions and maximizes contiguous free space in blocks.

See the PCTFREE setting for a table:

PCTUSED:is a block storage parameter used to specify when Oracle should consider a database block to be empty enough to be added to the freelist. Oracle will only insert new rows in blocks that is enqueued on the freelist.

For example, if PCTUSED=40, Oracle will not ADD new rows to the block unless sufficient rows are DELETED from the block so that it falls below 40% empty.

PCTFREE: is a block storage parameter used to specify how much space should be left in a database block for future updates.

For example, for PCTFREE=10, Oracle will keep on adding new rows to a block until it is 90% full. This leaves 10% for future updates (row expansion). 

When using Oracle Advanced Compression, Oracle will trigger block compression when the PCTFREE is reached. This eliminates holes created by row deletions and maximizes contiguous free space in blocks.

See the PCTFREE setting for a table:

PCTUSED:is a block storage parameter used to specify when Oracle should consider a database block to be empty enough to be added to the freelist. Oracle will only insert new rows in blocks that is enqueued on the freelist.

For example, if PCTUSED=40, Oracle will not add new rows to the block unless sufficient rows are deleted from the block so that it falls below 40% empty.