About Under-signs: explanation, restriction, exemption and distance: which statement is correct?

The under-signs '500 m' and '↑500 m↑' mean exactly the same thing: both simply indicate the distance to the main sign.
An under-sign with the word 'uitgezonderd' (excepted) plus a bicycle symbol means the traffic sign applies TO cyclists specifically, and only to them.
If an under-sign at an E-parking sign shows days and hours, the parking ban still applies all week — the under-sign is just extra information.
✓ An under-sign contains a further explanation OF THE TRAFFIC SIGN; without a main sign an under-sign has no independent effect.

Correct answer: An under-sign contains a further explanation OF THE TRAFFIC SIGN; without a main sign an under-sign has no independent effect.

Explanation

An under-sign contains a further explanation OF THE TRAFFIC SIGN; without a main sign an under-sign has no independent effect

'500 m' (no arrows) = applies over 500 metres from here; '↑500 m↑' (with arrows) = applies for the coming 500 metres — two different applications of the same number, not synonyms
'uitgezonderd' + symbol = the sign does NOT apply to that group (exemption); a bare symbol without 'uitgezonderd' restricts the sign TO that group instead
If an under-sign at E4 through E8, E12 or E13 shows days or hours, the orders/prohibitions apply ONLY during those days or hours — outside that window the ban does not apply