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.

WhyA yellow plate sign is used for messages that have no dedicated sign. It is an official sign, not an advertisement.