You are required to show the details of your purchases from composite taxable person in the heading under inward supplies liable to reverse charge as the purchases from composite taxable person is liable to tax under reverse charge.
Yes obviously he is registered in gst that is why he is composite taxable person The purchases from composite taxable person is liable to tax under reverse charge basis and the purchaser is liable to pay tax on such purchases you will get the credit of tax paid by you.
Reverse charge is applicable on your input the rate of tax which would otherwise be leviable if supplier is not registered as composite taxable person is applicable to you for payment of tax on such purchases
The simple reason being that existence of RCM is to ensure that tax that might get lost due to unregistered dealer’s trading, ultimately gets to government via registered dealer dealing with such unregistered dealer.
While when you make a purchase from composition opted dealer, you are not required to make any RCM based GST payment as it is borne by such composition opted dealer.