Question on Stored Procedure: taGetPurchReceiptNextNumber

I notice there are 3 parameters for this Stored Procedure: taGetPurchReceiptNextNumber in my company database.

I would like to know what the input parameter means here: @I_vInc_Dec

If I pass the input value as 0 or 1 I am getting the output @O_vPOPRCTNM (next receipt number)

if I leave it blank and execute the SP, the value @O_vPOPRCTNM returned is blank.

Can some one please let me know what the input parameter @I_vInc_Dec means here.

Thanks in advance..!