Noteer het aanslagnummer en het betalingskenmerk.

Noteer is the imperative form of noteren. It is used to give a command or instruction, like English write down or note.

In Dutch instructions, the imperative is very common:

• Lees de tekst.
• Vul uw naam in.
• Noteer het aanslagnummer.

Here, noteer sounds a bit formal and fits the style of official paperwork.

Because Dutch, like English, usually leaves out the subject in direct commands.

So:

• Noteer het aanslagnummer...

works the same way as English:

• Write down the reference number...

If you want to make it explicitly polite, Dutch can add u after the verb:

• Noteert u het aanslagnummer en het betalingskenmerk.

But on forms, letters, and official instructions, the shorter version without a subject is very common.

Yes, noteren is slightly formal and is very common in official or written instructions.

In everyday speech, many people would also say:

• Schrijf het aanslagnummer en het betalingskenmerk op.

That is more conversational.
So the difference is mainly register:

• noteer = more formal, official, written
• schrijf ... op = more everyday, spoken

Because Dutch forms compound nouns very freely, and they are normally written as one word.

So:

• aanslag
  • nummer → aanslagnummer
• betaling
  • kenmerk → betalingskenmerk

This is very typical Dutch. English often uses separate words where Dutch prefers one long compound.

Also, the last part is the main noun:

• an aanslagnummer is a kind of nummer
• a betalingskenmerk is a kind of kenmerk

Both are het-words.

In Dutch compounds, the article is usually determined by the final part of the compound:

• het nummer → het aanslagnummer
• het kenmerk → het betalingskenmerk

So even though aanslag- and betalings- add meaning, the grammatical gender usually comes from nummer and kenmerk.

Dutch normally repeats the article with coordinated singular nouns:

• het aanslagnummer en het betalingskenmerk

This sounds more complete and natural, especially in formal written Dutch.

Leaving out the second het is much less standard here. Dutch often prefers to mark each noun phrase clearly, especially in instructions and official language.

That -s- is a linking element used in many Dutch compounds.

The word is based on:

• betaling
  • kenmerk

but the compound becomes:

• betalingskenmerk

That -s- does not really add meaning by itself. It is just part of how many compounds are formed in Dutch. You will see the same kind of thing in other words too, such as:

• arbeidscontract
• levensmiddel

Unfortunately, this is not always fully predictable, so learners often have to get used to it word by word.

Yes. Aanslag can have different meanings depending on context.

In some contexts, it can mean something like attack or assault. But in administrative or tax language, it often refers to an official assessment or levy.

So in aanslagnummer, it has the official administrative meaning, not the everyday attack meaning. That is a useful reminder that Dutch compound words often depend heavily on context.