A more thorough explanation:

A letter of request is a legal document that asks a foreign court to perform a judicial act. This is usually done to seek assistance with taking evidence or serving process in a foreign jurisdiction. Letters of request are sent directly from the authority seeking assistance to the foreign nation’s Central authority.

For example, if a court case is happening in the United States, but evidence needs to be collected from a witness who lives in France, a letter of request can be sent to the French court asking them to help collect the evidence.

The letter of request must include specific information, such as the names and addresses of the parties involved, the necessary information regarding the proceedings, the evidence required, and the names and addresses of the persons to be examined if applicable. The letter must also be written in or accompanied by a translation of the language of the foreign authority.

Under the Hague Convention, the procedure for a letter of request has been simplified and its operation is not solely dependent on the principle of comity. Foreign signatories rarely refuse letters of request, except where a foreign nation does not consider evidence taking as a judicial function or when it believes that providing judicial assistance would usurp its national security or sovereignty.