Hi there
The official answer is...when there is less than 177 we block row 3,4 as all or almost all check-in luggage is in front hold, so havin 12 empty seats at the front balanced the A/c for take off n landing (to be honest its only nessesary in 1/100 occasions)
If there is less than 132 pax we should block first 6 and last 4 rows for the same reason but don't have to - its up to the dispatcher and take-off trim on a loadsheet.
Hope it answered your question
