Dimension Value problem

Hi,
I am new to NAV .
In purchase order, if i set line dimension value, On closing and reopening automatically another value is getting fetched from somwhere.The new value is generated automatically only for 1 particular dimension set ID .Workinf normally for other IDs.I am not able to find from where this value is getting populated.Can sum1 plz explain whats actually happening and from where the possibility of fetching a new value occurs???

Take a look at your Default Dimension settings, particularly in relation to the Vendor on the PO, and the various items (Inventory, G/L Account, etc) on the PO Lines. If you have set default values for these records, the defaults will be automatically applied to the order and/or lines.