It is a warning, instruction or prohibitory sign, which conveys a message that is not covered by the provisions of the road sign table

What is the meaning of this traffic sign?

1. It is a warning, instruction or prohibitory sign, for public transport only.
2. It is a square information sign.
3. Correct answer: It is a warning, instruction or prohibitory sign, which conveys a message that is not covered by the provisions of the road sign table.
4. It is an unofficial road sign, which conveys various optional guidances.