How Autoinvoice Import Program derives Accounting_RULE_DURATION for variable duration account type

Dear Experts,

Currently I am trying to create a Credit Memo for the original Invoice having variable accounting rule .

Custom code successfully populated, accounting_rule_duration in interface table ra_interface_lines_all table but post completion of auto invoice import program this field value is getting populated as NULL in ra_customer_trx_lines_all table.

Tried to serach in many sites  but couldn't find the solution for this cause.

Any pointers on this issue will be very hepfull.

Thanks in Advance!!