This gift was distributed when players purchase a Pokéball Plus for the usage with the games Let's Go Pikachu & Let's Go Eevee.
It is not known when they will stop bundling the mystery gift with the Pokéball Plus.
An internet connection is required to redeem the gift.
Format Ver.1.0.7-0, Post Updated Date:20190425_0654
What can be done with these files
PKM creation with PKHeX (drag & drop the wondercard file in the spot where you want the Pokémon)
In-Game Mystery Gift redemption with Switch Gift Data Manager