The FEIN (Federal Employer Identification Number) can be see as apart of this field "Tax Reporting Unit Name", but it doesn't exist as its own field.
Prior comment on: https://community.oracle.com/customerconnect/discussion/comment/804129#Comment_804129
The Tax Reporting Unit Name field in the Payroll Balances subject area has the FIEN appended to the end of the TRU. (Example: "NAME (xx-xxxxxxx)"
"Tax Reporting Unit"."Tax Reporting Unit Name"
I haven't found a field that just stores the FEIN number.