The necessary form and substance of an offer of proof at trial if a proponent of evidence is to preserve a claim for later review that the trial court erred in excluding evidence

Fed. R. Evid. 103(a) ("Rule 103. Rulings on Evidence (a) Effect of erroneous ruling. Error may not be predicated upon a ruling which admits or excludes evidence unless a substantial right of the party is affected, and (1) Objection. - In case the ruling is one admitting evidence, a timely objection or motion to strike appears of record,...If an opponent of evidence wishes to preserve a claim, for later review, that a trial court erred in admitting evidence, the opponent ordinarily must object to the offered evidence before the trial court rules that the evidence is admissible. If an opponent fails to make such an objection, the opponent will ordinarily be said to have waived or forfeited its claim that the trial court erred in admitting the evidence.

Fed. R. Evid. 801(c) advisory committee's note ("The definition follows along familiar lines in including only statements offered to prove the truth of the matter asserted. McCormick ? 225; 5 Wigmore ? 1361, 6 id. ? 1766. If the significance of an offered statement lies solely in the fact that it was made, no issue is raised as to the...If some human utterance has legal significance under applicable substantive law and the utterance is offered to establish that a verbal act having such legal significance has taken place, the utterance is not hearsay under Federal Rule of Evidence 801(c) or a "statement" for purposes of Federal Rule of Evidence 801(a)(2) even if the utterance also contains an assertion or appears to contain an an assertion.

United States v. Mitchell, 145 F.3d 572, 578 (3d Cir. 1998) (vacating conviction for conspiracy to commit and commission of Hobbs Act robbery, and use of and carrying a fiream during a crime of violence because the....A statement is admissible under the hearsay exception created by Federal Rule of Evidence 807 or a comparable State rule only if circumstantial evidence shows that the affected statement is as trustworthy as the statements that are made admissible by the hearsay exceptions created by Federal Rules of Evidence 803 and 804 or by comparable State rules.