Dutch term (edited): onderbord

see e.g.: https://en.wikipedia.org/wiki/Traffic_sign

"Categories

Traffic signs can be grouped into several types. For example, Annexe 1 of the Vienna Convention on Road Signs and Signals (1968), which on 30 June 2004 had 52 signatory countries, defines eight categories of signs:

A. Danger warning signs
B. Priority signs
C. Prohibitory or restrictive signs
D. Mandatory signs
E. Special regulation signs
F. Information, facilities, or service signs
G. Direction, position, or indication signs
H. Additional panels

[…]

Most warning signs display a black symbol on a white background within a red-bordered equilateral triangle. Since 2012, however, a more visibly distinctive design (taken from that used for school signs in the US) has been adopted for pedestrian-related signs: these consist of a fluorescent yellow-green pentagon with black border and symbol. Additional panels may be placed below signs to supplement their meanings."

(https://en.wikipedia.org/wiki/Traffic_sign )

====

"(1)
Permitted variants
1. “RED ROUTE” may appear on one line
2. Panels 7 and 8 shall be varied in accordance with Table 21
3. Panel 8 may be omitted
4. Additional panels may be added below Panel 8 to which Panel 8 symbols and legends may also be added"

("STATUTORY INSTRUMENTS, 2015 No. ROAD TRAFFIC, The Traffic Signs Regulations and General Directions 2015" @ https://assets.publishing.service.gov.uk/government/uploads/... )