This is an old question that is no longer in use.
Images may be missing and the information may be dated.
What is the significance of the white part of the red circles?
Red flashing light (SIG16)
”Stop. A vehicle may not pass the stop line or, in the absence of such, the signal.”
This is the way the Swedish Transport Agency has chosen to illustrate a flashing light, as it is impossible to make a light flash in a still image (the same images are used in paper brochures, for example).
There is also a driving licence book online (2022).
More examples of explanations: