About The seven categories of traffic signs (appendix 1): which statement is correct?
Command signs (round, blue) are a separate category from category C (round, red border).
A traffic sign with a command or prohibition is a strong recommendation, not a hard requirement.
The overtaking-ban sign (F1) belongs to category C, because it is a round sign with a red border like the other prohibition signs.
The priority-road sign (B1) is a triangle, just like a warning sign.
Explanation
trap: shape-without-colour reflex. Command signs (round, blue) are a separate category from category C (round, red border). Round only tells you 'command or prohibition', not which of the two