What does an auxiliary plate beneath a bridge warning sign showing 'H = 3.6 m' mean?

1. The minimum vertical clearance beneath the overhead obstacle is exactly 3.6 metres (Svijetli otvor / visina prolaza)
2. The length of the bridge is 3.6 km
3. The river depth under the bridge is 3.6 metres
4. The bridge can support a maximum weight of 3.6 tonnes