The proceeding in which the evidence will be used is significant because some countries only grant assistance for certain kinds of prosecutions (e.g., offenses that are also crimes in the country from which assistance is requested); others exclude assistance for specific categories of cases (e.g., tax violations, military and political offenses).
One caveat about purpose: some countries limit assistance to the purpose stated in the request. Once such a country grants assistance for that purpose, as a general rule you will not be able to use the evidence for another reason without the express permission of the country that provided it. Contact the Office of International Affairs for guidance in securing such permission.
Finally, the stage of the proceeding for which aid is needed may also be significant since some countries may grant assistance only after the filing of formal charges.
[cited in USAM 9-13.512]