Pli:

06:45 Apr 8, 2020 

R712-1 du Code (français) de la propriété intellectuelle : « La demande d'enregistrement de marque est déposée au siège de l'Institut national de la propriété industrielle ou y est envoyée par pli postal ou par tout mode de télétransmission dans les conditions définies par décision de son directeur général... » (https://www.legifrance.gouv.fr/affichCodeArticle.do?idArticl... Check similar codes from other French-speaking countries.
As for English, they say "by post" in the UK: "Fill in the paper forms if you want to apply by post." (https://www.gov.uk/how-to-register-a-trade-mark/apply). I've also seen "by mail" on the brand registration websites of other English-speaking countries.
That should be OK unless of course they have their own way of saying it in patentese - which would be hardly surprising.

03:52 Apr 8, 2020 

Writeaway is almost certainly right and it is ridiculous to wait for "a precise legal term from a more experienced translator".

katt = cat, hund = dog and pli (French) = cover/letter

It's that straightforward

00:15 Apr 8, 2020 

Sorry, I meant to say Linguee.

00:13 Apr 8, 2020 

Of course, I checked Proz and I checked three dictionaries, as well as Lingo and DeepL, before asking the question. My best approach is that there is likely some metaphorical connection with a folder or envelope and I was and still am hoping to get the precise legal term from a more experienced translator.

21:30 Apr 7, 2020 

Check a Fr-En dictionary and/or the glossary for the French word "pli"