Bekreftelsen gjelder ettersendingen.

The ending -en is the definite singular ending for many masculine and feminine/common-gender nouns in Bokmål.

• en bekreftelse = a confirmation

• bekreftelsen = the confirmation

• en ettersending = a forwarding / a resending
• ettersendingen = the forwarding / the resending

So Norwegian often puts the at the end of the noun instead of using a separate word like English does.

Because Norwegian usually expresses definiteness by adding it to the noun itself.

In English:

• the confirmation
• the forwarding

In Norwegian:

• bekreftelsen
• ettersendingen

That is one of the most important differences from English noun structure.

Gjelder is the present tense of å gjelde, and in this kind of sentence it usually means:

• concerns
• applies to
• is about

So Bekreftelsen gjelder ettersendingen means that the confirmation is connected specifically to the forwarding/resending.

A few useful forms:

• å gjelde = to concern / to apply to
• gjelder = concerns / applies to
• gjaldt = concerned / applied to
• har gjeldt = has concerned / has applied to

Because the basic word order here is:

• subject + verb + object

So:

• Bekreftelsen = subject
• gjelder = verb
• ettersendingen = object

This is normal Norwegian sentence structure, just like English:

• The confirmation concerns the forwarding.

Because å gjelde normally takes a direct object in sentences like this.

So Norwegian says:

• Bekreftelsen gjelder ettersendingen.

Not:

• Bekreftelsen gjelder for ettersendingen in this meaning

English often uses different structures such as concerns, applies to, or is about, but Norwegian simply uses gjelder + object here.

Ettersending is a noun formed from:

• etter- = after / onward / following
• sending = sending

In practice, ettersending is often used for mail forwarding, resending, or forwarding something to a new address.

So in many real-life contexts, the sentence could be about a postal forwarding arrangement or some kind of administrative forwarding service.

The dictionary forms are the indefinite singular forms:

• bekreftelse
• ettersending

When learning vocabulary, you would usually memorize them with the article:

• en bekreftelse
• en ettersending

Then the definite forms are:

• bekreftelsen
• ettersendingen

In Bokmål, bekreftelse is very commonly treated as a common-gender noun, so you will usually see:

• en bekreftelse
• bekreftelsen

The same goes for ettersending:

• en ettersending
• ettersendingen

In practice, for a learner, the safest choice is to use the en forms.

Sometimes, yes, because gjelde can also mean be valid for or be applicable to, depending on context.

For example:

• Tilbudet gjelder ut måneden. = The offer is valid until the end of the month.
• Regelen gjelder alle. = The rule applies to everyone.

But in Bekreftelsen gjelder ettersendingen, the most natural sense is usually concerns / applies to / refers to, rather than physical validity in a narrow sense.

This sentence sounds fairly formal and administrative, which is very normal in letters, emails, and official notices.

A more explicit version might be:

• Denne bekreftelsen gjelder ettersendingen. = This confirmation concerns the forwarding.

Adding denne makes it clearer that you mean this confirmation specifically.

Without denne, the original sentence is still completely natural, especially in official writing.