Federal Rule of Evidence 106 is called the "rule of completeness." It attempts to prevent an unfair portrayal of a piece of evidence when presenting the full piece of evidence would paint a clearer picture. Different courts have interpreted its application in different ways. Professor Miller has posted a piece on this explanation and I wrote a post on the Fourth Circuit/SC:
As Professor Miller explained, there is a split among courts about how to apply 106. Is it a procedural mechanism that merely applies to when a party can introduce the other half of the otherwise admissible evidence? Or is it a substantive mechanism that allows a party to introduce an otherwise inadmissible piece of evidence in order to be fair and paint the entire picture?
It appears that the Second Circuit is of the "substantive mechanism" group. In US v. Moore (a summary order, not a published opinion), the defendant was convicted of transporting cocaine through the airport. She gave a statement to the police admitting she knew the cocaine came from a drug dealer, however, she wanted to introduce other portions of the statement and lay witness testimony explaining that she had "cognitive deficits."
The court excluded portions of her statement to law enforcement, because they held that they were not necessary nor were they probative. However, in their explanation the court seems to hold that if the statements were probative/necessary, then they would have been admitted under 106. While it is not exactly clear if this falls under substantive or procedural 106 - I think their reasoning follows a substantive framework.
United States v. Moore, No. 18-2414-CR, 2019 WL 5681219 (2d Cir. Nov. 1, 2019)
The court in Moore cites to another Second Circuit Case - U.S. v. Johnson. From this case, I am confident that the Second Circuit does indeed follow the substantive framework for 106:
United States v. Johnson, 507 F.3d 793 (2d Cir. 2007)