Når det gjelder dekningsgraden, velger de sammen.

Here Når det gjelder is a very common fixed expression meaning when it comes to, as for, or regarding.

So even though når often means when, this whole expression is not really about time. It is more like a topic marker:

• Når det gjelder været ... = As for the weather ...
• Når det gjelder pris ... = When it comes to price ...

It is best learned as a chunk.

The det is part of the fixed expression Når det gjelder. You usually do not translate it word-for-word in English.

Literally, gjelder means concerns / applies to, so the phrase is something like when it concerns it, but that literal breakdown is not very useful in normal English. For learners, it is better to remember:

• Når det gjelder + noun phrase = when it comes to + noun phrase

This is because of Norwegian V2 word order in main clauses.

When something comes first in the sentence—here, Når det gjelder dekningsgraden—the finite verb must come in the second position. So the order becomes:

• Når det gjelder dekningsgraden, velger de sammen.

Not:

• Når det gjelder dekningsgraden, de velger sammen.

This is very common in Norwegian:

• I dag kommer han.
• På skolen snakker vi norsk.
• Når det gjelder dette, er jeg enig.

So the fronted phrase takes first position, then the verb comes next.

Yes, structurally it comes from a subordinate-clause pattern, but in practice many learners benefit from treating Når det gjelder as a fixed introductory expression.

The important practical point is this:

• it comes first,
• it sets the topic,
• and the main clause that follows has inversion: velger de.

So whether you think of it as a subordinate clause or as a set phrase, the result in the sentence is the same.

Dekningsgraden is the definite form of dekningsgrad.

• dekningsgrad = coverage level / coverage rate (indefinite)
• dekningsgraden = the coverage level / the coverage rate (definite)

In this sentence, the speaker is referring to a specific coverage level, so the definite form is natural.

Also, Norwegian often uses the definite form where English might be a little less explicit. So even if English says something like coverage level in a general-looking way, Norwegian may still prefer dekningsgraden depending on context.

It is a compound noun:

• dekning = coverage
• grad = degree / level / rate

Together:

• dekningsgrad = coverage rate / degree of coverage / coverage level

Then the definite ending is added:

• dekningsgrad + en = dekningsgraden

This is very typical Norwegian word-building: several ideas are packed into one compound noun.

Sammen means together.

So velger de sammen means that they choose together or they make the choice jointly.

Its placement is natural here after the verb and subject:

• velger de sammen

If the sentence did not begin with the fronted phrase, you could have:

• De velger sammen.

So sammen is just telling you how they choose: jointly, not separately.

Sometimes, yes, depending on context.

Norwegian present tense often covers both:

• simple present: choose
• present progressive: are choosing

So velger can mean:

• choose
• are choosing

The exact English translation depends on the wider situation. Norwegian does not require a separate form like English are choosing.

Here de means they.

Norwegian de can sometimes also appear as those / the ones in other contexts, but in this sentence it is clearly the subject pronoun they, because it comes after the verb in the normal inverted word order:

• velger de sammen = they choose together

The comma separates the introductory part from the main clause:

• Når det gjelder dekningsgraden, velger de sammen.

This makes the sentence easier to read and is very common after a fronted clause or long introductory phrase. It shows that Når det gjelder dekningsgraden is setting up the topic, and then the main statement follows.