What is the difference between the following signs?

1. Sign 505 indicates a stop for private buses. Sign 506 indicates a Tramway stop.
2. Sign 505 indicates a Tramway Stop. Sign 506 indicates a stop for Public bus transport.
3. Correct answer: Sign 505 indicates a stop for Public Transport (Buses). Sign 506 indicates a stop for a Public Tramway.
4. Sign 505 indicates a public transport stop on a Motorway (Freeway). Sign 506 indicates a public transport stop.

Why505 is a stop for public buses, 506 is a tram stop. Do not mix them up; they have different symbols.