For Unit Holders (UH)
If an SPV is paying tax on its profit under Section 115BAA at the lower rates, the profit distributed to unit-holders(UH) by way of dividend through business trust, would not be exempt from tax (i.e taxed in hands of UH)
If SPV does not opt for 115BAA then SPV shall be paying tax at higher rates on its profit so when such SPV further distributes dividend to UH through Business Trusts , it has been exempted in the hands of Unit holders u/s 10(23FD)
For Business Trusts (REIT/INVIT)
To qualify as a REIT, the trust must distribute at least 90% of its taxable income to shareholders. In turn, REITs typically don't pay any corporate income taxes because their earnings have been passed along as dividend & Interest payments.
Hence No matter SPV opts 115BAA or doesnt opt for 115BAA , Dividend income from SPV is exempt to Business Trust in both cases u/s 10(23FC) unlike unit holders.