Hello,
FM means Function module, if you have any specific requirement for auto UD then you can take a help of an ABAPr and design accordingly by using the FM. Already i have suggested you the same. BAPI is also available for auto UD process.
And if your requirement is stdn only then use the stdn functionality of SAP by setting the customizing Auto UD at Plant level, Create job variant and execute in backgrnd for auto UD processing.
Regards