Freelists, PCTUSED, PCTFREE, Row chaining, Row migration

Segments(table for example)
    Extents (initial extent and next extents)
             Blocks (set of OS blocks):header(block address,type of segment), table directory, row directory, freespace(PCTUSED, PCTFree)
                       rows: rowheader,now of columns, cluster key, rowid, column length,column date
   
Freelists : list of free blocks where data is inserted, decided by PCTFREE
PCTFREE: is a percentage of space in a block upto which the rows will be inserted lests say if 20% is set as pctfree then upto 80% of block is used for inserts and till that time the block will be found in freelist. once it reaches that 20% pctfree limit it means it has consumed all 80% and reached the pctfree percentage level in that case from freelist this block is removed and is used for updates.
PCTUSED: the block is used for updates once it is inside pctfree and this update may cause increase in block size usage or decrease in its usage due to updates and deletes and once the usage brings the space limit down and down and down to given PCTUSED limit the block will be back to freelist. Hence PCTUSED is a limit which says that due to updates if block is getting free and it reaches to a given limit of
freeness in its space then it can again be used for inserts.
Row Migration: due to updates many a times size of row does not fit into a block and it requires to be allocated into completely new block leaving address of that block into old block for index traversal. This concept of allocating the updated row to a new block due to its non fitness in current block after update is called as row migration.
Row Chaining: when a row can not fit at all into a block due to its size which is greate than the size of block in those cases oracle splits the rows into blocks this concept of spliting the rows into blocks is called as chaining. This may happend due to lengthy columns been used in table definition like long or long raw or char(lengthy size).
Extent: group of blocks
select SEGMENT_NAME,bytes,blocks(BLOCKS GIVEN),BLOCK_ID 'FROM BLOCK' from dba_extents where segment_name = 'TEST';