What does this traffic sign indicate?

You chose correctly that this sign indicates a federal road (Bundesstraße). In Germany, yellow rectangular signs with black numbers are used to mark federal roads, which are important main roads connecting cities or regions. The number shown is the specific road number.

This sign does not indicate any speed limit or minimum speed requirement. Speed limits would be displayed with different signs, typically circular with a red border. So, the sign simply tells drivers they are on federal road number 35, helping with navigation and route identification.