What is basis in retirement plan

Basis would be an amount for which you did not receive a tax deduction when you contributed it.  For federal purposes, contributions to a plan like a 401k were deductible, meaning they reduced the income on which you paid tax.  For PA, though, that income used for contributions was taxable, so you had a basis in PA for those contributions.  Hopefully, your 1099-R shows employee contributions.  If not, you would have to get the information from the plan fiduciary.

Here's an article to read about it.

https://revenue-pa.custhelp.com/app/answers/detail/a_id/1470/related/1/session/L2F2LzEvdGltZS8xNTIxM...

If you  are a PA resident, your basis is the sum of all your contributions you made into that particular retirement plan through the years less the sum of all your distributions made  from that retirement plan through the years.