Ibase determination in Service order

Hi,

I am creating a service order from service contract as a followup via Z function module.  I have populated the header and item details but i get a error with respect to ibase.

"Item 10: please enter a ibase for each item".

How do i determine the ibase progamatically?? Once i find out the ibase i would then use "crm_order_maintain" to maintain them. But Currently am not sure which function module would determine me the ibase details. Any help is really appreciated.

Thanks

Abishek