If a Data Item is already included on an Earnings Data Item Basis for line, it will not be available for use on the Employee's Additional Basis for line. 

Remove the Data Item from the Earnings Data Item Basis for line to enable the Data Item for use on the Employee's record. 

In order to put a tax on the additional basis for line on the accounting information tab, it has to be off ALL earnings involving the type (salary, hourly or other). The best way to check this out is to run a detailed historical data item print.  Use the binoculars to search on the tax you are working with.  In Model Divison 95, .SWT CO is on multiple earning items. It has .SWT CO listed on the basis for field (field 4) of both SALARY and REG SAL, which both pull .S AMT.  Once .SWT CO is removed from those, then it is available to put on the Salary selection on the accounting information tab.  If it is removed from every earning pulling .H AMT, then it is available for use on the Hourly selection. The 3rd type is Other, so anything not pulling .H AMT or .S AMT falls into that category. 

Note: After changing the Historical Data Items you will have to log out and back into FundWare before the changes are recognized on the Employee screens.