According to J 16/84 (OJ 1985, 357, headnote), which concerned the application of the former Art. 7(2) RFees 1973 (now Art. 6(2) RFees), if when a fee is paid the purpose of the payment has evidently been given incorrectly, this deficiency is not prejudicial if the intended purpose can be established without difficulty from the remaining information. The inadvertent use of a fee by the EPO for a different purpose from that evidently intended by the person making the payment has no effect on the purpose intended by that person.
In J 19/96 the board noted that J 23/82 (OJ 1983, 127) held that the indication of the purpose of a payment within the time limit for payment was not a mandatory requirement for payment to have been made in due time and, according to Art. 7(2) RFees 1973 (now Art. 6(2) RFees), could thus still be given later. However, the current board doubted whether this meant that it was generally possible to change the purpose of a payment after expiry of the relevant time limit with retroactive effect to the date on which the payment was made.
Both J 23/82 and J 19/96 concerned the indication of the purpose of fee amounts paid in respect of designation fees for individual contracting states under the earlier law (applicable to European patent applications filed and international applications entering the European phase before 1 April 2009). They are dealt with from this perspective in the "Case Law of the Boards of Appeal of the EPO", 6th edition 2010, Chapter VI.F.2.4.