I'm not a lawyer, but this has been explained to me in the following way.

The text seems to be describing the actual physical bundle of documents pertaining to a rogatory request, which is when one legal system asks another for evidence generated within the latter's system to use as evidence within the former's, generally documentation, (perhaps also a witness to come testify, but that may be a Hague Convention request instead). This request is generally known in English as a "letter rogatory."

For example, French authorities issue a letter rogatory to Swiss authorities for copies of interrogations conducted in Switzerland of French subjects originally indicted for crimes in France that have transnational aspects that have gotten them indicted in Switzerland as well (money laundering, for example), because the information obtained by the Swiss also concerns the crimes committed in France.

So, based on your context above, letter a) is the folder containing all the requesting documentation, and b) is the folder that would contain all the documents/legal instruments possibly received from the foreign legal system.

Therefore, I used folder rather than dossier in my suggested translation, as dossier is more abstract in my mind, but perhaps there is a better way to express the actual, physical bundle of paper than that.

Good luck!