invoice refrence field

Hi

while posting docuement in invoice refrence field when i  pressed F1 it shows that for that special rules applies to creditmemo which has "V" in field .

can any 1 upadte me is there any setting to it ? as i am running standard doc and want to run F.13 but not able to clear against credit memo as it shows "V" in invoice refrence field .

For refrence i am ataching screenshot

Regards

-Kaushal