Der Erstattungsantrag ist insofern vollständig, als er alle Belege enthält.

Erstattungsantrag is a compound noun:

• Erstattung = reimbursement, refund
• Antrag = application, request

So Erstattungsantrag is literally something like reimbursement application or refund request.

A useful rule in German is that the last part of a compound noun determines the gender. Since der Antrag is masculine, der Erstattungsantrag is also masculine.

Because der Erstattungsantrag is the subject of the sentence, so it is in the nominative case.

The basic structure is:

• Der Erstattungsantrag = the reimbursement application
• ist = is
• vollständig = complete

So the sentence begins with the thing that is complete, which makes it the subject.

Because vollständig is a predicate adjective, not an adjective directly in front of a noun.

Compare:

• der vollständige Antrag = the complete application
  
  • here the adjective comes before the noun, so it takes an ending
• der Antrag ist vollständig = the application is complete
  
  • here it comes after sein, so it does not take an ending

That is very common in German.

This is a fairly formal construction meaning insofar as, to the extent that, or in that respect that.

It does not simply say the application is complete in every possible sense. It qualifies the statement. The idea is:

• the application is complete insofar as it contains all the supporting documents

So the sentence sounds careful and precise, almost administrative or legal.

Because the meaning is slightly different.

• weil gives a straightforward reason:

  • Der Erstattungsantrag ist vollständig, weil er alle Belege enthält.
  • The application is complete because it contains all the supporting documents.

• insofern ... als is more limited and precise:

  • Der Erstattungsantrag ist insofern vollständig, als er alle Belege enthält.
  • The application is complete insofar as / to the extent that it contains all the supporting documents.

With insofern ... als, the speaker is being more cautious. It suggests: this is the respect in which it can be called complete.

No. Here als is not the comparison word you know from sentences like:

• größer als = bigger than

It is also not the time-related als meaning when in past events.

In this sentence, als is part of the fixed formal construction insofern ... als.

So you should understand it as part of one expression, not as a separate familiar als meaning than.

Because als er alle Belege enthält is a subordinate clause.

In German, subordinate clauses:

• are usually separated by a comma
• send the conjugated verb to the end

So:

• er alle Belege enthält
• subject: er
• object: alle Belege
• verb at the end: enthält

This is standard German subordinate-clause word order.

Because German pronouns follow grammatical gender, not natural gender.

• der Antrag is masculine
• so the pronoun referring back to it is er

This does not mean the application is male in any real-world sense. It is just grammatical agreement.

English usually uses it for things, but German often uses:

• er for masculine nouns
• sie for feminine nouns
• es for neuter nouns

Alle Belege is accusative plural, because it is the direct object of enthält.

• enthalten = to contain
• What does it contain? alle Belege

As for meaning, Belege can mean:

• receipts
• supporting documents
• vouchers
• documentary proof

In administrative contexts, Belege often means the documents that prove or support a claim.

Because here the phrase is accusative plural, and alle takes the form alle in that case.

So:

• nominative plural: alle Belege
• accusative plural: alle Belege

The form allen would be used in other contexts, for example dative plural:

• mit allen Belegen = with all supporting documents

So in this sentence, alle Belege is exactly the right form.

It is natural, but it sounds formal and somewhat bureaucratic or administrative.

Words and structures that make it formal include:

• Erstattungsantrag
• insofern ... als
• Belege
• the careful, qualified tone

In everyday conversation, a speaker might choose something simpler, for example:

• Der Antrag ist vollständig, weil alle Belege dabei sind.
• Der Antrag ist komplett, er enthält alle Belege.

So the original sentence is perfectly good German, but it belongs more to official or written language than casual speech.

Yes. Insofern can also be used on its own, often meaning something like in that respect, to that extent, or insofar.

For example:

• Insofern hast du recht. = In that respect, you are right.

But in your sentence, the pattern insofern ... als is a set structure introducing the specific point that justifies the statement.

So both are possible in German, but here the two-part construction is what creates the formal meaning.