Hi Matthew,

Thanks for posting in the community!

From your description, you can't add more level then 8 in the Excel not
only for the row but also the column.

Excel can only supports at most 8 level for the outlining. So far as I
know, this is the same in Excel 2000, XP and 2003.

Please feel free to let me know if you have any further question. Our
pleasure to be of assistance.

Have a nice day!

Wei-Dong Xu
Microsoft Product Support Services
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