Partnership.  They don't engage in any activities subject to sales tax.  They have no employees.  You use the Federal Tax ID to pay the PE Tax (it says so right on the website).  There is no longer a Business Entity Tax.  The question arose this year because I'm getting an error message in ProSeries for NOT entering a CT Tax Registration Number.  I've been able to file in prior years without the number.  Like I said, I spoke to a DRS rep and they confirmed that clients of this type (two people who jointly own a rental property) do NOT need a CT Tax Reg #.