Use of Customer number in FI12 for ACH Payments

Hello Friends - We are sending Domestic ACH Payment files in UK to HSBC through Idocs (Payment medium program specified in Payment Methods in Country is RFFOUS_T, Data set – ACH).

I see that Customer number in FI12 is filled with some number. I have gone through the program documentation RFFOUS_T and do not see the need for filling this field. 

Filling this field is as per Bank’s requirement? Pls throw me some idea on this.

Thanks