What is meaning of SSZ in 10053 trace on 12.2

select count(*) from tab1;***************************************BASE STATISTICAL INFORMATION***********************Table Stats::  Table: TAB1  Alias: TAB1  (Using composite stats)  #Rows: 41126836  SSZ: 0  LGR: 0  #Blks:  0  AvgRowLen:  792.00  NEB: 0  ChainCnt:  0.00  ScanRate:  0.00  SPC: 0  RFL: 0  RNF: 0  CBK: 0  CHR: 0  KQDFLG: 1select count(*) from tab1 partition(p10);***************************************BASE STATISTICAL INFORMATION***********************Table Stats::  Table: TAB1  Alias: TAB1  Partition [10]  #Rows: 20480777  SSZ: 20480777  LGR: 0  #Blks:  2406080  AvgRowLen:  788.00  NEB: 0  ChainCnt:  0.00  ScanRate:  0.00  SPC: 0  RFL: 0  RNF: 0  CBK: 0  CHR: 0  KQDFLG: 1

Cannot find any help on SSZ?

Also, why is the SQL in partition giving the right number of blocks vs the one against the full table says #Blks: 0

Thanks,

Vikram R