Answer:
A yellow arrow in the graphic shows the location selected in the result table. This can be a support node, as in your case, or a grid point that is selected in the result table in RFEM.
If you display it using the Tables menu item and click, for example, in a row in results table 4.1, a yellow arrow is displayed for this node in the graphic so you can find this point more quickly.
The arrow appears after opening the file because a row was selected in the result tables when you closed the file the last time, and this setting was saved.