In colloquial English, you say "Not that I know of" in reply to a yes-no question when you think the answer is in the negative but you are not 100% sure. I was wondering about the function of "that" in this expression. What is this? Is this the same 'that' (a subordinate conjunction) as in "I know that he is coming to the party"? Or is it the same as 'that' (a relative pronoun) in "This is the person that I talked to you about"? Or is it something else?
Thank you in advance for your help.