Update hook
Use this hook to (re) determine the value of the field based on the current record values. Think of determining defaults and calculating derived values.
Example:
function extern tdsls401.pric.update(long mode)
{
select txprc100.pric
from txprc100
where txprc100.item = :tdsls401.item
as set with 1 rows
selectdo
tdsls401.pric = txprc100.pric
endselect
}