von Belang sein
to be relevant
To be important, relevant, or of significance.
Deine Meinung ist für diese Entscheidung nicht von Belang.
Your opinion is not relevant to this decision.

concern / relevance
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to be relevant
To be important, relevant, or of significance.
Deine Meinung ist für diese Entscheidung nicht von Belang.
Your opinion is not relevant to this decision.

The word 'Belang' (concern/relevance) belongs to formal language. It is mainly used in written, official, or administrative contexts.
In dieser Angelegenheit sind rechtliche Belange zu berücksichtigen.
Legal concerns must be taken into account in this matter.
In the meaning of 'Wichtigkeit' (importance), the word is used almost exclusively in the fixed expression 'von Belang sein' (to be relevant/of importance). It then functions like the adjective 'wichtig' (important).
Diese Information ist für unsere Planung von großem Belang.
This information is of great importance for our planning.
In the meaning of 'Anliegen' (concerns) or 'Interessen' (interests), the plural 'die Belange' (the concerns/interests) is almost always used. It is often preceded by whose interests are meant, e.g. with the genitive or 'von'.
Die neue Chefin will sich zuerst um die Belange der Mitarbeiter kümmern.
The new boss first wants to take care of the employees' concerns.
important
'wichtig' and 'von Belang' both describe that something has significance for a matter. 'wichtig' is the normal everyday word and fits almost any time something matters or needs to be paid attention to. 'von Belang' sounds more formal and is often used in discussions about decisions, rules, or precise information. You would more often say 'Das ist wichtig' (That is important), but in a report or during an assessment, 'Das ist von Belang' (That is relevant) sounds more natural.

Für die Entscheidung ist diese Information von Belang.
This information is relevant to the decision.
Am Morgen ist die kurze Besprechung für das Team wichtig.
In the morning, the brief meeting is important for the team.