В случае если код не указан, нужен сопроводительный документ.

В случае если is a more formal way to say if or in the event that.

• Если код не указан... = If the code is not specified...
• В случае если код не указан... = In the event that the code is not specified...

In everyday speech, если is much more common.
В случае если is typical of official, legal, administrative, and bureaucratic language.

Because в случае если код не указан is a subordinate clause, and it is separated from the main clause by a comma.

Structure:

• subordinate clause: В случае если код не указан
• main clause: нужен сопроводительный документ

So the comma works like the comma in English in a sentence like If the code is not specified, a supporting document is needed.

Указан is the short passive participle of the verb указать.

• указать = to indicate, specify
• указан = specified / indicated

So код не указан literally means the code is not specified.

This is a very common official style pattern in Russian:

• адрес указан = the address is specified
• срок не указан = the срок is not specified
• номер указан неверно = the number is specified incorrectly

Because не указан is a short-form participle used as part of the predicate, while не указанный is a full-form participle used more like an adjective before a noun.

Compare:

• Код не указан. = The code is not specified.
  Here не указан is the predicate.

• Не указанный в заявке код... = The code not specified in the application...
  Here не указанный describes the noun.

So in your sentence, Russian needs the short form: код не указан.

Because код is the grammatical subject of не указан.

In код не указан:

• код = subject, nominative singular
• не указан = predicate

This is like:

• документ подписан
• номер не указан
• адрес изменён

The thing being described remains in the nominative.

Because it agrees with документ, which is masculine singular.

• документ = masculine singular
• therefore: нужен

Compare:

• нужна справка — a certificate is needed
• нужно письмо — a letter is needed
• нужны документы — documents are needed

So in your sentence:

• нужен сопроводительный документ

Because нужен does not take a direct object here. Instead, it works like a predicate word meaning is needed.

So the structure is basically:

• сопроводительный документ нужен = a supporting document is needed

Both нужен and сопроводительный документ are part of the predicate structure, and the noun is in the nominative as the subject.

Compare:

• Мне нужен паспорт. = I need a passport.
  Literally: A passport is needed to me.
• Нужен документ. = A document is needed.

So this is not like I need a document with a normal accusative object in English.

Because Russian normally has no present-tense copula in sentences like this.

English says:

• a supporting document is needed

Russian says:

• нужен сопроводительный документ

The present tense is is simply omitted.

Compare:

• Он врач. = He is a doctor.
• Документ готов. = The document is ready.
• Код не указан. = The code is not specified.

Yes, absolutely.

• Если код не указан, нужен сопроводительный документ.
• В случае если код не указан, нужен сопроводительный документ.

Both are correct, but the second is more formal and more typical of written instructions or regulations.

A learner should recognize that в случае если often signals official style rather than everyday speech.

Yes. Сопроводительный is a regular adjective modifying документ.

• документ = document
• сопроводительный документ = accompanying / supporting / covering document

The adjective agrees with the noun:

• masculine singular nominative: сопроводительный документ

This is just standard adjective-noun agreement.

Yes. Russian word order is flexible, though some versions sound more natural in certain contexts.

For example:

• В случае если код не указан, нужен сопроводительный документ.
• Если код не указан, нужен сопроводительный документ.
• Сопроводительный документ нужен, если код не указан.

All of these are grammatically possible.

Your original sentence sounds natural in formal written Russian because it starts with the condition and then gives the requirement.

Because указать is a standard formal verb meaning to indicate / specify / state. It is very common in forms, instructions, and regulations.

Compare the tone:

• указать код = to specify the code
• написать код = to write the code

In administrative language, Russian often prefers указать, because the idea is not just physically writing something, but officially providing or stating it.

Yes, and it would sound even more formal or impersonal.

For example:

• В случае если код не указан, требуется сопроводительный документ.

This means almost the same thing: a supporting document is required.

Nuance:

• нужен = needed
• требуется = required

Both are common in formal contexts, but требуется sounds more bureaucratic and stricter.

Yes, very much so.

Several features make it formal:

• в случае если instead of plain если
• не указан in passive-style official wording
• сопроводительный документ
• нужен in an impersonal administrative statement

This is the kind of sentence you would expect in:

• instructions
• forms
• regulations
• application guidelines
• customs or shipping documents

A native speaker would understand it easily, but it does not sound especially conversational.