Defaulted Uom 'AU' in Blanket Purchase Order

Hi Colleagues,

Does anyone know how and why the unit of measure 'AU' for 'activity unit' is defaulted into the purchase order line item for a blanket text purchase order?

Even If I enter 'EA' during the manual line item entry this is overwritten with 'AU' when I press enter and then it is no longer changeable....

Any advice as to where this default would occur from? Especially confused as this is obviously text based, not material master based

Thanks

Steve