In previous releases you always had to enter a tax indicator for one-time postings for rental agreements (transaction FOB1), even if the G/L account defined by the flow type was not relevant to tax.
It is now no longer necessary to fill the 'tax indicator' in such cases.
The system issues a warning for G/L accounts that are relevant to tax but do not necessarily require a tax indicator. You may suppress this warning if you wish (transaction OBA5), the same way as for F1 error messages.