This is what we have been taught to use by "the IP law and law firm" (our main client).

Bardehle Pagenberg uses this translation as well, a pretty renowned IP law firm, also based in Munich, Germany's intellectual property capital and home to the DPMA/German Patent and Trademark Office (GPTO).

A protective brief is filed in court cases (so AFAIK it's not an out-of-court dispute/settlement thing) and is to do with petitions for the granting of a preliminary injunction (the Court's "einstweilige Verfügung" ordering that the other party urgently cease and desist from infringing the IP right of the party petioning the granting of a preliminary injunction).

For example, in the protective brief the party filing it might argue that the matter (their ceasing and desisting from infringing the other party's IP right) isn't urgent because ...

The aim of the protective brief is to get the point across that the preliminary injunction petitioned by the other party is unfounded.

It is more than a letter, it is a statement vis-a-vis the Court (= hence the "brief/Schriftsatz" part).

On another note, there are (well, basically) two different notations (by convention) that law firms use in regard to their cases:
- giving the Plaintiff first.
- giving the law firm's client first, i.e either Plaintiff or Defendant (law firm in this case meaning the law firm filing the protective brief)

To quote from the reference:
"Remedies of the defendant

Protective brief

After the defendant has received a cease-and-desist letter, but before a potential request for preliminary injunction has been filed, the defendant has, as a precautionary
measure, the opportunity to file a so-called ‘protective brief ’ (‘Schutzschrift’) in every competent district court. This way, and in addition to (or instead of) a simple letter of reply, the defendant can provide the courts with all necessary facts and arguments if he
believes that a potential request for preliminary injunction is unfounded. Most district courts are linked to an online register which has been implemented for this
purpose, but some courts, eg in Berlin, Munich and Cologne, are not linked to this database, even though they are of some significance, so request papers have to
be filed separately with them via fax and regular mail.

A protective brief may include any relevant arguments, such as lack of urgency or lack of a claim on the merits.

In order to refute urgency, the defendant may for example provide arguments that the applicant was actually aware of the alleged infringement for a lengthy period of time, usually more than one month, thereby rebutting the presumption that the matter is
urgent. However, for positive knowledge it is not sufficient simply to show that the applicant could have known of the alleged infringement earlier; rather, it is usually necessary to show that the persons actually responsible for pursuing intellectual property matters in the company and/or the managers positively became aware of the infringement, eg by email or in person, or that they could not possibly have overlooked a potential advertisement with the alleged infringement. If prior knowledge is proved, the entire motion for preliminary injunction will be rejected. In cases of doubt,
it rests with the discretion of the court to issue summons for an oral hearing in due course, for example where there are contradictory facts."