Statutory immunity, also known as formal immunity, should be distinguished from informal immunity. The latter term, often referred to as "pocket immunity" or "letter immunity," is immunity conferred by agreement with the witness. For example, the government and a cooperating defendant or witness might enter into a plea agreement or a non-prosecution agreement if the defendant or witness agrees to cooperate. Testimony given under informal immunity is not compelled testimony, but is testimony pursuant to an agreement and thus voluntary. The principles of contract law apply in determining the scope of informal immunity. United States v. Plummer, 941 F.2d 799, 802 (9th Cir. 1991); United States v. Britt, 917 F.2d 353 (8th Cir. 1990), cert. denied, 498 U.S. 1090; United States v. Camp, 72 F.3d 759 (9th Cir. 1996) [replacing 58 F.3d 491 (9th Cir. 1996)]. Grants of informal immunity that do not expressly prohibit the government's derivative use of the witness's testimony will be construed to prohibit such derivative use. Plummer, supra. But a grant of informal immunity that expressly provides for derivative use of the testimony by the government will be upheld. United States v. Lyons, 670 F.2d 77, 80 (7th Cir. 1982), cert. denied, 457 U.S. 1136.
An important difference between statutory/formal immunity and informal immunity is that the latter is not binding upon the States. This follows from the fact that the local prosecutor representing the State is normally not a party to the agreement between the witness and the Federal prosecutor, and thus cannot be contractually bound by the Federal prosecutor's agreements.
[cited in USAM 9-23.100]