A criminal subpoena is a legal document issued by a court, with which an individual is ordered to appear in court to provide testimony or a piece of evidence. The document has a very high level of authority and is the equivalent of being directly summoned to court by authority officials. Criminal subpoenas can vary slightly depending on the particular circumstances related to the individual's appearance in court, but they always require the presence of the person receiving the subpoena in court.
The most common type of criminal subpoena involves summoning an individual to court for the purpose of providing testimony - however, in some cases the individual may be required to appear at an attorney's office instead, never going into actual court. This is used in cases where the individual is only serving an auxiliary purpose in the legal proceedings and their presence during the hearing isn't required.
In the second common case, the individual is summoned due to their possession of specific documents or other items of evidence that can relate to the case they're being summoned to. In this case, the individual may not be required to present themselves in court, but they are obliged to provide the complete set of items that have been requested in the subpoena, and the subpoena may specify additional conditions related to those items that have to be met by the individual. Failure to turn in all of the items can result in prosecution similar to not responding to a regular subpoena.