What does it mean when a prosecutor offers you immunity in exchange for your testimony about a crime or a potential crime?
It depends. Generally speaking, the prosecutor may offer you transactional immunity or use immunity. The difference between them is significant.
Transactional immunity is the broadest kind of immunity you can be offered. With transactional immunity in place, you're protected from prosecution over any issue related to your testimony. It does not, however, protect you from prosecution for crimes that were unrelated to your testimony.
For example, imagine that you worked for a doctor that was selling prescriptions. You're given transactional immunity for your testimony, during which you admit that you helped the doctor falsify Medicare records to hide his illegal sales. The prosecutor couldn't turn around and charge you based on that admission.
Use immunity, on the other hand, is much more specific and narrowly defined. With use immunity, prosecutors only agree not to use your testimony against you. However, they can still prosecute you for crimes that you mention if they obtain evidence of those crimes from another source.
For example, imagine that you have to use immunity to testify against the doctor about his pill selling. You mention your assistance with Medicare fraud. Prosecutors were already aware, however, that you had committed that fraud. While prosecutors couldn't use your testimony against you, you also wouldn't enjoy any particular immunity from charges.
Immunity is a powerful item in the prosecution's toolbox, but it doesn't always confer the benefits that witnesses think it does. If you're being offered any kind of immunity for your testimony, it's incredibly important that you understand exactly what you are getting -- and under what conditions -- before you agree to testify. An experienced criminal defense attorney can help you understand your options and protect your rights.