Dear Martin
"BAL_Y" returns the Opening Balance for the Current Year. This is a calculated field. (Current Balance on File - Sum of Net Posted for the Year). This is the correct code to use for Financial Reports
"OPEN" returns the value in the Opening Value Bucket from the physical file. This is a Value direct from the disk with no calculation, (it will be the same as reported by our Report Writers). This, most likely, should not be used for Financial Reports.
The 2 values (.OPEN and .BAL_Y) will be the same unless a Prior Year Posting has been applied against the account.
Because of your question, R&D has decided to change the operation of .OPEN to be the same as that of .BAL_Y as there would be no real reason to want the value currently returned by .OPEN on a financial statement. 
Thank you!
[This message has been edited by Andrew Bates (edited 10-01-2001).]