Triangle as line number

You can choose that 'triangle' which is displayed instead of line number.
I can take a screenshot if needed.

My question is - what is it used for?

The triangle was used for special lines like fairground shuttles or excursion tours to Berlin's forests and lakes.

What is the code for the triangle or where can you find it?

On the matrix displays, enter LINIE/KURS "00002". On the rollsigns, you'll find the three triangle parts at the very bottom below the number "9".