The T indicates Taxpayer.
When working on a Married Filing Joint return you'll get a pop up asking if you want to enable TSJ indicators. This allows you to assign entries to a specific person on the return if you wanted to do any comparisons for MFJ vs MFS etc.
They won't impact the calculation at all.
https://proconnect.intuit.com/community/form-1040/help/using-the-tsj-indicators/00/5814 shows you how to use them.
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