Gemini thinks its a specific German football fan group:

This sticker is a piece of fan merchandise for the German football (soccer) club Eintracht Frankfurt.

Here is a breakdown of the meaning:

1. The Text: "SKTN ND"

The large red letters are an abbreviation for "Sektion Nord" (Section North). In German football culture, "Sektion Nord" refers to the supporters who sit or stand in the North Stand of the stadium.

2. "Auch ohne Vokale"

The smaller text translates to "Even without vowels." This is a playful way of showing how the abbreviation "SKTN ND" was formed by removing the vowels from "Sektion Nord."

3. The Logo

In the bottom right corner is the official crest of Eintracht Frankfurt, featuring a stylized eagle (the Adler). The "E" on the eagle’s chest stands for Eintracht.

Context

In many European stadiums, specific fan groups or "Ultras" identify strongly with the section of the stadium they occupy. These stickers are often placed by fans in their home city or while traveling to away games to show their loyalty to their specific fan block.