MODIFYALL is the high-volume version of the MODIFY function. If we have a group of records in which we wish to modify one field in all of these records to the same new value, we should use MODIFYALL. It is controlled by the filters that are applied at the time of invoking. The other choice for doing a mass modification would be to have a FIND-NEXT loop in which we modified each record one at a time. The advantage of MODIFYALL is that it allows the developer and the system to optimize code for the volume update. Any system optimization will be a function of what SQL statements are generated by the C/AL compiler.
The syntax for MODIFYALL is as follows:

Record.MODIFYALL (FieldToBeModified,NewValue 
  [,TriggerControlBoolean ] )

The TriggerControlBoolean value, a TRUE or FALSE entry, controls whether or not the table's OnModify trigger fires when this MODIFY occurs. The default value is FALSE, which would result in the field OnValidate trigger not being executed. In a typical situation, a filter or series of filters would be applied to a table followed by the MODIFYALL function. A simple example where we will reassign all the TerritoryCodes for a particular Salesperson to NORTH is as follows:

Customer.RESET ; 
Customer.SETRANGE("Salesperson Code",'DAS'), 
Customer.MODIFYALL("Territory Code",'NORTH',TRUE);